d20-magic

What this skill does

# Spellcasting Skill

Provides guidance for handling spellcasting in d20 system adventures. Use this skill when players cast spells, manage spell resources, or when you need to resolve magical effects.

## Core Formulas

These formulas are essential for all spellcasting. The spellcasting ability depends on class: Intelligence (Wizard), Wisdom (Cleric, Druid, Ranger), or Charisma (Bard, Paladin, Sorcerer, Warlock).

### Spell Save DC

When a spell forces a target to make a saving throw:

```
Spell Save DC = 8 + Proficiency Bonus + Spellcasting Ability Modifier
```

**Example**: A level 5 Wizard with Intelligence 18 (+4 modifier) has proficiency bonus +3.
- Spell Save DC = 8 + 3 + 4 = **15**

### Spell Attack Modifier

When a spell requires an attack roll against a target:

```
Spell Attack Modifier = Proficiency Bonus + Spellcasting Ability Modifier
```

**Example**: Same level 5 Wizard with Intelligence 18.
- Spell Attack Modifier = 3 + 4 = **+7**

To make a spell attack, Use the mcp__corvran__roll_dice tool:

```
Use the mcp__corvran__roll_dice tool:
{ "groups": [{ "n": 1, "d": 20 }], "modifier": 7 }
```

Compare the result against the target's Armor Class (AC).

## Spell Slots

Spell slots represent a caster's magical energy. Each level 1+ spell requires expending a slot of that level or higher.

### Key Rules

1. **Expending Slots**: Casting a level 1+ spell uses one slot of the spell's level or higher
2. **Recovery**: All expended slots return after a Long Rest
3. **Upcasting**: Spells cast with higher-level slots often deal more damage or have enhanced effects
4. **One Spell per Turn**: You can only expend one spell slot per turn (cantrips don't use slots)

### Slot Availability

See `references/spellcasting.md` for complete spell slot tables by class and level.

**Full Casters** (Bard, Cleric, Druid, Sorcerer, Wizard): Access spell levels 1-9, gain slots at every level.

**Half Casters** (Paladin, Ranger): Access spell levels 1-5, gain first slot at level 1 (Ranger) or level 2 (Paladin), progress more slowly.

**Warlocks**: Use Pact Magic with fewer slots that recharge on Short Rest. See references for details.

## Cantrips

Cantrips are level 0 spells that don't consume spell slots.

- **At-Will Casting**: Can be cast unlimited times
- **Damage Scaling**: Damage cantrips increase at levels 5, 11, and 17
- **No Upcasting**: Cantrips cannot be cast at higher levels

**Cantrip Damage Scaling Example** (Fire Bolt):
| Character Level | Damage |
|-----------------|--------|
| 1-4             | 1d10   |
| 5-10            | 2d10   |
| 11-16           | 3d10   |
| 17+             | 4d10   |

## Concentration

Many powerful spells require concentration to maintain their effects.

### Concentration Rules

1. **One at a Time**: You can only concentrate on one spell at a time
2. **Duration**: The spell description specifies maximum concentration time
3. **Voluntary End**: You can end concentration at any time (no action required)

### Breaking Concentration

Concentration ends immediately if:

- **New Concentration Spell**: You cast another spell requiring concentration
- **Damage Taken**: You must make a Constitution saving throw to maintain concentration
- **Incapacitated**: You gain the Incapacitated condition
- **Death**: You die

### Concentration Saving Throws

When you take damage while concentrating:

```
Constitution Save DC = Higher of: 10 OR (Damage Taken / 2)
```

Maximum DC is 30 (from 60+ damage).

**Example**: A Wizard concentrating on Haste takes 18 damage.
- DC = max(10, 18/2) = max(10, 9) = **DC 10**

Roll the concentration save:

```
Use the mcp__corvran__roll_dice tool:
{ "groups": [{ "n": 1, "d": 20 }], "modifier": 2 }
```

(Assuming Constitution save modifier of +2)

If the save fails, the spell ends immediately.

## Spell Attack Procedure

When a spell requires an attack roll:

1. **Calculate modifier**: Proficiency Bonus + Spellcasting Ability Modifier
2. **Roll attack**: Use the mcp__corvran__roll_dice tool with `1d20+[modifier]`
3. **Compare to AC**: Meet or exceed target's AC to hit
4. **Roll damage**: If hit, roll damage dice specified by the spell

**Example**: Casting Chromatic Orb (ranged spell attack, 3d8 damage)

```
# Attack roll with +7 modifier
Use the mcp__corvran__roll_dice tool:
{ "groups": [{ "n": 1, "d": 20 }], "modifier": 7 }
# If result >= target AC, roll damage
Use the mcp__corvran__roll_dice tool:
{ "groups": [{ "n": 3, "d": 8 }] }
```

## Saving Throw Spells

When a spell requires the target to make a saving throw:

1. **Announce the DC**: Use caster's Spell Save DC
2. **Identify ability**: The spell specifies which ability (Dex, Con, Wis, etc.)
3. **Target rolls**: Roll 1d20 + target's saving throw modifier
4. **Resolve effect**: Spell description states what happens on success/failure

**Example**: Fireball (Dex save, DC 15)
- Target has Dexterity save +3
- Roll: 1d20+3 = 14 (fails, takes full 8d6 fire damage)
- If rolled 15+: Success, takes half damage

## Ritual Casting

Spells with the Ritual tag can be cast without expending a slot.

### Ritual Rules

- **Extended Time**: Takes 10 minutes longer than normal casting time
- **No Slot Used**: Doesn't expend a spell slot
- **Must Be Prepared**: Most classes require the spell to be prepared
- **No Upcasting**: Ritual casting uses the spell's base level only

Classes with ritual casting: Bard, Cleric, Druid, Wizard (from spellbook without preparation).

## Spellcasting Components

Spells require specific components to cast:

| Component | Requirement |
|-----------|-------------|
| **V** (Verbal) | Must speak incantations; cannot cast if silenced |
| **S** (Somatic) | Must have free hand for gestures |
| **M** (Material) | Must have specified material or spellcasting focus |

**Focus Substitution**: A spellcasting focus (arcane focus, holy symbol, druidic focus) can replace material components that don't have a gold cost and aren't consumed.

## Quick Reference

| Situation | Formula |
|-----------|---------|
| Spell Save DC | 8 + Proficiency + Ability Mod |
| Spell Attack | 1d20 + Proficiency + Ability Mod |
| Concentration DC (damage) | Max(10, Damage/2), cap 30 |

## References

For detailed spell slot tables and class-specific spellcasting features, see:
- `references/spellcasting.md` - Complete spell slot progression tables
- `references/srd/07_Spells.md` - Full spell descriptions
- `references/srd/03_Classes/` - Class-specific spellcasting rules