Chapter 01 · Core Rules
Base Resolution
Philosophy
In Cerberus, you describe what your character tries to do and how they approach it. The GM judges the situation, taking their attributes, history, equipment, and plan into account. When the outcome is certain, play moves on without a roll.
When the outcome is uncertain, three types of roll resolve the action: a prescribed chance roll, an opposed check, or a saving throw. The GM judges, and the dice decide.
Attributes
Every character has six attributes, expressed directly as modifiers: Strength (STR), Dexterity (DEX), Constitution (CON), Intelligence (INT), Wisdom (WIS), and Charisma (CHA). A character with STR +8 adds 8 when a roll calls for Strength.
Roll No. 1—The Prescribed Chance Roll
To resolve an uncertain task, such as climbing a wall or picking a lock, the GM announces the character's chances of success. These depend on their attributes, history, equipment, and the player's proposed plan.
One convention, always from the player's perspective: the GM announces “X chances out of Y.” The player rolls 1dY. On a result of X or lower, the player gets what they wanted.
GM: Climbing that wall at night, with your agility and rope: 4 chances out of 5. Player: (rolls 1d5) I rolled 2. GM: You clear the wall without a sound.
Three golden rules for the GM:
- Do not roll when the outcome is certain. A master thief picking an ordinary lock succeeds. A peasant trying to jump a 30-foot chasm fails. Roll only when genuine uncertainty exists.
- The plan matters more than the character sheet. Waiting for the guard shift, creating a diversion, or crossing the roof improves the odds.
- Announce the odds before the roll, never after. The stated probability is a contract. A player can withdraw after hearing the odds, but cannot negotiate once the die has been cast.
Roll No. 2—Opposed Checks
An opposed check directly compares two conflicting forces. Each side rolls a d20 and adds the indicated value. The higher result wins. The effect that caused the check specifies what happens on a tie.
The most common opposed checks are:
| Check | Formula |
|---|---|
| Reflex check | 1d20 + Reflex modifier against 1d20 + Reflex modifier |
| Caster level check | 1d20 + CL against 1d20 + CL |
| DC check | 1d20 + DC against 1d20 + DC |
A character's Reflex modifier equals DEX plus the level bonus assigned by their class: full level for a good Reflex Save, half level for a standard Reflex Save, rounded down.
The Reflex Duel (Priority)
When two actions conflict in time—who strikes first, whether the spell fires before the arrow—each side makes a Reflex Save. The highest result acts first. On a tie, roll again.
Roll only when the order matters, after the actions have been declared. Combat has no fixed turn order or initiative roll at its start. See Reactive Combat for competing actions and the bonus for declaring first.
Roll No. 3—Saving Throws
When the world acts on a character, they resist with one of three Saves:
| Save | Formula | Resists |
|---|---|---|
| Reflex | 1d20 + Reflex modifier | anything avoided by moving |
| Will | 1d20 + WIS + class level bonus | illusions, fear, mental manipulation, invisibility |
| Fortitude | 1d20 + CON + class level bonus | poison, disease, exhaustion, transformation |
Compare the Save with the effect's DC. For a spell, DC = 10 + CL/2 + the caster's magic attribute. For an environmental danger, the GM sets the DC; see the Environment chapter. The defender wins a tie. For each Save, the class adds full level for a good progression or half level, rounded down, for a standard progression.
Advantage and Disadvantage
Some feats, items, or circumstances grant Advantage on a roll: roll twice and keep the better result. Disadvantage is its mirror: roll twice and keep the worse result. With Advantage, a Will Save rolled at 7 and 15 keeps the 15.
Rounding Convention
When a calculation produces a fraction, retain it through every step. Round the final result down once.
Round each independent value separately. Round a DC, derived attribute, or number of slots when calculating it. For damage, retain fractions while applying bonuses, percentages, critical hits, and DR, then round down the final damage of each attack.
With CL 5 and INT +8: 10 + 5/2 + 8 = 20.5, resulting in DC 20.
Attacks: No Roll to Hit
An attack that is neither dodged nor parried hits automatically. Roll damage directly. The defender can dodge or parry with a weapon or shield. Armor reduces the damage by its Damage Reduction (DR). See Martial Combat and Defenses.
Chapter Summary
- General uncertainty → prescribed chance roll (“X out of Y”; low is success).
- Direct conflict → opposed check. Reroll ties in a Reflex duel.
- The world acts on you → Save against DC.
- Attack → direct damage, mitigated by dodge, parry, and DR.