Achtung! Cthulhu — 2d20 System

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