Achtung! Cthulhu

The Secret War

It is World War II. The whole world is burning. But behind the headlines, a Secret War rages — between Allied occult intelligence services and Axis cultists, sorcerers, and mad scientists who have learned the truth of what lurks at the edges of reality.

You are an agent: a soldier, scholar, spy, mechanic, or occultist recruited because you saw something you weren't supposed to see, survived something nobody else did, or asked the right question in the wrong library. You fight for Section M (British) or Majestic (American), alongside French resistance and the brave peoples of every occupied nation.

The enemy is not just Nazis. It's the Black Sun, a sorcerous arm of the SS. It's Nachtwölfe, their weird-science rivals. It's the Deep Ones in the Atlantic. It's the alien Mi-Go in the mountains. It's Nyarlathotep, the Crawling Chaos, and behind him, dread Cthulhu himself.

This is pulp horror. The odds are stacked, but your agent is brave, clever, and lucky. Heroes make a difference here. Embrace the cliches. Deliver the one-liner. Make the once-in-a-million shot. Then punch evil right in the tentacles.

Factions

How The Game Works

The 2d20 System is designed to feel like a pulp movie. You and your fellow players collaborate on a story. The GM presents situations, plays all NPCs, and decides when the dice come out. You roll dice whenever success is uncertain and interesting.

There are only a few concepts to remember to play:

• Attributes (1–14): raw potential — Agility, Brawn, Coordination, Insight, Reason, Will.
• Skills (0–5): trained ability — Academia, Athletics, Fighting, etc.
• Focuses: specialisations inside a skill (e.g. Handguns, Occultism, Urban Survival).
• Truths: short phrases describing who you are or the scene around you.
• Talents: special rules-bending abilities your agent has earned.
• Three currencies: Momentum (yours), Threat (the GM's), Fortune (personal luck).

The Dice

You will use two kinds of dice:

d20 — Skill Dice

Twenty-sided dice. You roll 2d20 to resolve most tests. You can buy up to 3 more d20s (max pool 5d20).

Challenge Dice (CD) — Damage & Effect

Challenge Dice are specially-marked six-sided dice. If you only have normal d6s, read them like this:

An Effect triggers every special ability listed on a weapon or spell.

Making a Test

1. The GM tells you which Attribute + Skill to use. Add them together — that's your Target Number (TN).
2. The GM sets a Difficulty (0–5). Most are 1.
3. Roll 2d20 (or more if you've bought extra).
4. Count successes:
   • Each die ≤ TN = 1 success.
   • If you have a relevant focus: each die ≤ your skill value = 2 successes.
   • Each natural 1 = 2 successes.
   • Each natural 20 = a Complication.
5. If successes ≥ Difficulty → you pass. Any leftover successes become Momentum.

Buying Extra Dice

Before you roll, you can buy up to 3 extra d20s. Each one costs Momentum:

• First extra d20: 1 Momentum
• Second: 2 Momentum
• Third: 3 Momentum

You can also pay Threat to the GM to buy dice instead. The GM loves this — it gives them ammo later — but sometimes it's the right call.

Assisting

Another agent can roll 1d20 at their own TN to help. Their successes stack with yours if you score at least one. Describe how you're helping — a hand up a cliff, a word of encouragement, a cover fire burst.

Momentum — Your Resource

Momentum is a group pool shared by all players. Max 6 at any time.

You earn Momentum from excess successes on passed tests (one per extra success). You can spend it on any test, on anyone's turn.

Common Spends

Threat — The GM's Resource

Opposite to Momentum, the GM has Threat. They start each session with 2 Threat per player and gain more when:

• You choose to pay Threat instead of Momentum (to buy dice, or to buy off a complication).
• The scene becomes dangerous or an alarm is raised.
• An NPC generates unspent Momentum — it converts 1:1 to Threat.

The GM spends Threat to empower NPCs, summon reinforcements, change truths, or make scenes worse. It's a natural tension mechanic — sometimes the right move is to feed the GM in exchange for a critical advantage now.

Fortune — Pulp Luck

Each agent starts each mission with 3 Fortune (max 5). You may spend 1 per non-combat scene and 1 per round in combat.

Use Fortune for moments that matter. Don't hoard all of them.

Truths

A Truth is a short phrase that describes something about your agent, an NPC, or the scene. Your agent starts with at least four — drawn from Archetype, Background, Language, and Nationality.

Mechanically, a Truth can:

• Lower a test's Difficulty by 1 when it clearly helps.
• Raise it by 1 when it clearly hinders.
• Make something possible or impossible.

You can create new truths during play — by spending 2 Momentum, by spending Fortune (Make It Happen), or when the narrative clearly supports it. Truths can also change or be removed over time.

Combat in a Nutshell

Combat is tense but fast. The GM picks who starts. Turns alternate PC ↔ NPC until all have acted, then the round resets.

Zones & Range

On Your Turn

• 1 Major action (the big thing — Attack, Cast a Spell, Rush, Create Truth, etc.).
• 1 Minor action (Aim, Draw Item, Move, Prepare).
• You can spend 1 Momentum for an extra Minor, or 1 Fortune (once/scene) for an extra Major.

Attacking

1. Declare target + weapon.
2. Roll the skill test:
   • Melee — Agility + Fighting, opposed by target's same roll.
   • Ranged — Coordination + Fighting.
   • Mental — Will + Academia, opposed by target's Will + Resilience.
3. If you hit, roll the weapon's Challenge Dice for stress.
4. Subtract target's Armour + Cover (or Courage + Morale for mental).
5. Apply remaining stress; check for injuries.

Cover

Use it. A sturdy object gives Cover 2 (stacked with Armour). Rolling behind something is usually a minor action.

Ammo

You don't count bullets. You have 3 Ammo as a currency used for Salvo shots that add special effects (Area, Stun, Vicious…). Regular shots never run out.

Stress, Injuries & Healing

Your agent has a stress track. Take damage, fill it up.

Injuries Trigger When

• You take 5+ stress from a single source (after Armour/Cover).
• Your stress track fills up.
• You take more stress while already full.

Each injury raises your Complication Range by 1. 3 injuries (in any combination of physical/mental) → you are Defeated and out of the scene. If your injuries are mostly physical, you're also dying until stabilised.

Healing

• Stress heals at the end of each scene with time to rest.
• Injuries persist until treated — someone with Medicine skill can heal you.
• Defeat can be avoided by spending Fortune.

Magic in the Mythos

If your agent is a spellcaster, you have a Power rating and a tradition:

• Traditional — inherited folk magic (e.g. Celtic, Runeweaver). Uses Insight.
• Researcher — self-taught from forbidden tomes. Uses Reason. Powerful but dangerous.
• Dabbler — you stumbled into it. Uses Will. Flawed spells, limited knowledge.

Casting a Spell

1. Prepare your mantle at the start of a day (Diff 0 test) — hold spells up to your Power.
2. In combat: Prepare (minor action), then Cast a Spell (major action).
3. Roll the spell's skill test. Pay the cost (mental stress) after, whether you succeed or not.

Building Your Agent

Your agent — "agent" is the in-world term for a PC — is built from ABC + details:

Archetype

The core class. Pick one:

Background

What you did before the war: Academic, Athlete, Criminal, Entertainer, Journalist, Physician, Police, Spiritual Leader, etc. Shapes your skills and contacts.

Characteristic

A wildcard trait that marks you out: Bookworm, Dilettante, Dreamwalker, Street Kid, The Lucky One, Young at Heart… Often ties to how you got pulled into the Secret War.

Putting It Together

Pick an Archetype first. Add a Background — even an unexpected one. Add a Characteristic that contradicts or colours them. The interesting agents are the ones whose pieces don't obviously fit.

Truths & Name

Derive at least four truths from your ABC + nationality + language. Pick a name, a look, a voice, quirks. How does your agent move, talk, dress? What do they carry that they'd never lose?

Pregens

If your GM hands you a pregen — great. Read the truths and talents. Read the quotes. Picture the person. Then make them yours: give them a voice, a phrase, a habit. The sheet is a starting point, not a cage.

At The Table

Session Zero

Before you play, the group talks about tone, limits, and comfort. Achtung! Cthulhu involves WWII, cultists, body horror, and psychological dread. Not every table wants every topic. Be honest about what you want and don't.

Safety Tools

• Veils — content that happens "off-camera." Fade to black.
• Lines — topics that are completely off-limits.
• X-Card — tap the card (or a hand, or say "pause") to cut a scene. No justification needed.

Collaboration & Pulp Tone

Spotlight Etiquette

• Everyone gets a moment to shine. If you've been rolling all scene, toss the ball to someone else.
• Describe what your agent is doing, not just the mechanical result.
• Use Momentum for the team. Pay Threat sparingly but dramatically.
• When you roll a complication, volunteer what goes wrong. You know your agent best.

Quick Cheat Sheet

What is the 2d20 System?

Achtung! Cthulhu uses Modiphius's 2d20 System, a cinematic, pulp-driven engine designed for fast decisions and two-fisted action. Unlike strict simulationist systems, 2d20 leans on narrative currencies — Momentum, Threat, Fortune — that let players and the GM push scenes in dramatic directions.

Your agent (PC) is defined by six attributes, twelve skills, optional focuses for specialisation, truths that describe who they are, and talents that bend the rules.

Characters

Each agent is defined by:

The Six Attributes

The Twelve Skills

Focuses

A focus is a narrower specialisation inside a skill (e.g. Fighting (Handguns), Academia (Occultism), Observation (Sight)). When the focus applies, dice that roll at or under the skill value score two successes instead of one — effectively crits on your skill ranks.

Truths, Talents & Power

• Truths — short phrases that describe the agent (Veteran Commando Captain, Searching for Arcane Knowledge). Truths can raise or lower difficulty, or make things possible/impossible. Start with at least four from Archetype, Background, Language, Nationality.
• Talents — special rules-bending abilities granted by training, class, or experience.
• Power — spellcaster rating (1–3). Only spellcasters have this.

Skill Tests

Whenever an outcome is in doubt, make a skill test.

1. GM picks the appropriate Attribute + Skill. The sum is the Target Number (TN).
2. GM sets a Difficulty (0–5). Most ordinary tests are Difficulty 1.
3. Player rolls 2d20 (up to 5d20 if they buy extra dice).
4. Every die ≤ TN = 1 success.
   • If a relevant focus applies: every die ≤ the skill value = 2 successes.
   • Every natural 1 = 2 successes (automatic crit).
   • Every natural 20 = a complication.
5. If successes ≥ Difficulty → pass. Excess successes become Momentum.
6. GM narrates outcome. Player may spend Momentum to enhance. Then apply complications.

Improving the Odds (Buying Extra d20s)

Maximum dice pool: 5d20.

Difficulty & Success

Success at Cost

For tests where failing outright doesn't fit the story, the GM may allow success at cost: the test passes, but adds one automatic complication. No Momentum can be spent on a test resolved this way.

Momentum

Momentum is the players' shared currency. Every excess success above Difficulty becomes 1 Momentum. The group pool caps at 6; excess is lost.

Common Momentum Spends

Threat

Threat is the GM's currency. The GM starts each adventure with 2 Threat per player.

How the GM gains Threat

• Threatening circumstances (hostile environment, alarm raised).
• Unspent NPC Momentum converts to Threat at 1:1.
• Players pay Threat (2 per complication) to buy off a complication.
• Players can pay Threat instead of Momentum for any Momentum spend (this is a trade the GM wants).

How the GM spends Threat

• NPC Momentum spends (all the player options).
• Buy off NPC complications (2 Threat).
• Change/create a Truth (2 Threat).
• Summon reinforcements: Trooper 1, Lieutenant 2, Nemesis 3.
• Trigger environmental effects or narrative changes.

Fortune

Each agent starts each mission with 3 Fortune, max 5. One Fortune spend per non-combat scene; one per round in combat.

NPCs do not have Fortune.

Truths & Complications

Truths

A Truth is a short phrase — a fact about a character, scene, or environment. It can:

• Reduce or increase a test's Difficulty by 1 (or more with a numeric rating like Smoke 2).
• Make an action possible or impossible.
• Give the GM flexibility to bridge rules and narrative.

Complications

A Complication is a negative truth introduced by:

• Rolling a natural 20 on a test.
• GM spending Threat.
• Choosing Success at Cost.

Buying off: players may pay 2 Threat to cancel a complication before it fires.

Complication Range

Normally complications trigger on a 20. The GM can raise the range (e.g. 19–20, up to +4) for especially risky situations. Injuries also increase a character's complication range by 1 until treated.

Opposed Tests

Both sides roll an appropriate skill test (base Difficulty 1). Resolve by:

• Active succeeds, Reactive fails → Active wins.
• Active fails, Reactive succeeds → Only Reactive's effect resolves.
• Both fail → Active fails, Reactive gains nothing extra.
• Both succeed → Compare Momentum. Highest wins, but loses 1 Momentum per point the opponent scored. Loser spends nothing. Ties go to the Active — but Active loses all their Momentum.

Action & Conflict

Zones & Distances

Firing outside a weapon's ideal range adds +1 Difficulty per band off.

Rounds & Turns

• The GM picks who starts (usually a player).
• Play alternates sides (PCs ↔ NPCs) until all have acted, then a new round begins.
• Each turn: 1 major + 1 minor action (plus free talk/small gestures).

Ammo

Each agent starts with 3 Ammo. Ammo is currency for Salvo attacks (which add Area, Stun, Vicious, etc. on the weapon profile). Regular shots don't deplete Ammo.

Major & Minor Actions

Minor Actions (1 per turn, +1 for 1 Momentum)

Major Actions (1 per turn; +1 via 1 Fortune, once/scene)

Attacks, Stress & Injuries

Attack Sequence

1. Declare target and weapon.
2. Skill test:
   • Melee — Agility + Fighting, Diff 1, opposed by target's Agility + Fighting Diff 1.
   • Ranged — Coordination + Fighting, Diff 1 (+ range modifiers).
   • Mental — Will + Academia, Diff 1, opposed by target's Will + Resilience Diff 1.
3. Inflict stress: roll the weapon's Challenge Dice (CD).
4. Subtract Armour + Cover (physical) or Courage + Morale (mental).
5. Apply remainder to target's stress track; check for injuries.
6. If target won an opposed melee, they may inflict stress back OR move within Close range.

Challenge Dice (CD)

Injuries

You suffer 1 injury if:

• A single attack/hazard deals 5+ stress after reduction.
• Your stress track is filled.
• You take more stress with a full track.

Multiple conditions at once = multiple injuries.

• Each injury: +1 Complication Range until treated.
• 3 injuries total (any mix) → Defeated.
• Defeated + more physical than mental injuries → Dying. Roll 1 CD per dying turn (+1 each subsequent turn); if 2+ Effects ever roll on a single roll, you die.

Attribute Bonus Challenge Dice

When rolling damage, some attacks add CDs based on a relevant attribute:

Resistance, Armour, Cover, Morale

• Armour — worn protection rating. Reduces physical stress.
• Cover — terrain-based rating (Cover 1, Cover 2…). Stacks with Armour against physical attacks.
• Courage — the character's intrinsic resolve. Reduces mental stress.
• Morale — temporary boost from allies/inspiration. Reduces mental stress.

Going Prone

• Ranged attackers at >Reach: +1 Difficulty against you.
• Attackers within Reach: –1 Difficulty against you.
• You can't Rush until you stand up (a minor action).

Weapon Effects

Triggered when a CD rolls an Effect (5 or 6). All listed effects of the weapon apply when any Effect rolls.

Fatigue & Healing

Fatigue

Each point of Fatigue reduces maximum stress by 1. At 0 max stress: unconscious (defeated). More Fatigue while unconscious: death.

Remove with Brawn + Resilience or Will + Resilience Diff 1 — removes 1 Fatigue plus 1 per Momentum spent.

End-of-Scene Healing

Unless defeated, agents recover all stress at the start of a new scene with sufficient rest. Someone with Medicine can also run a Reason + Medicine test — pick the difficulty; on a success, heal X CDs of stress where X = chosen Difficulty.

Magic

Magic comes in two flavours:

• Battlefield magic — shorter, combat-oriented spells stored in the caster's mantle.
• Ritual magic — larger, more potent; used to summon, communicate with, or permanently shape reality.

Types of Spellcaster

A caster can push Power to 3 by adding 1 Threat per Effect rolled.

Preparing the Mantle

Before using battlefield spells, the caster prepares a mantle holding spells equal to their Power rating. Preparing is a Diff 0 test, taking minutes = sum of chosen spells' difficulties. Mantle holds until the caster sleeps or is knocked unconscious.

Casting a Battlefield Spell

1. Prepare (minor action).
2. Cast a Spell (major action) — roll the test; attribute depends on caster type (Insight / Reason / Will).
3. Pay the spell's cost (CDs of mental stress) after resolving the test, whether or not it succeeded.

Complication range increases by the spell's Difficulty — Diff 2 spell suffers a complication on 19 or 20. Complications on spells can be spectacular; but if the spell succeeded, the effect still triggers.

Counterspell (Free Reaction)

Once per round, when an enemy you can see casts a spell: roll your Power in CDs. Each Effect rolled raises their spell's Difficulty by +1. A caster who counterspells may not cast a spell on their own next turn.

Ritual Magic (Simplified)

Rituals have a stress track and steps. Each successful casting test rolls Power-many CDs as "damage" to the ritual track. Injury-triggering conditions (5+ stress in one roll; track filled; etc.) complete a step. When all steps are complete, the ritual fires. Cost is paid after every roll, pass or fail.

Quick Reference Cheat Sheet