Skill

d20-monsters

This skill should be used when the GM needs to create NPCs or monsters, generate stat blocks, determine enemy stats, design creatures, or generate adversaries for d20-style RPG encounters. Provides SRD 5.2 stat block format and NPC templates.

Install

npx claudepluginhub rjroy/adventure-engine-corvran --plugin d20-system

Tool Access

This skill uses the workspace's default tool permissions.

Preview

Create monsters, NPCs, and adversaries using the SRD 5.2 stat block format. This skill provides the standard format, reference tables, and templates for generating balanced creatures.

Supporting Assets

references/npc-example.mdreferences/npc-template.mdreferences/stat-blocks.md

SKILL.md

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Monster and NPC Creation Skill

Create monsters, NPCs, and adversaries using the SRD 5.2 stat block format. This skill provides the standard format, reference tables, and templates for generating balanced creatures.

When to Use This Skill

This skill applies when:

Creating a new monster or NPC for an encounter
Converting a narrative concept into game statistics
Generating stat blocks for adversaries
Balancing creatures for specific Challenge Ratings
Looking up stat block format requirements

Stat Block Format Overview

Every creature stat block follows this structure:

Name and General Details: Name, size, type, tags, alignment
Combat Highlights: AC, HP (with hit dice), Speed, Initiative
Ability Scores: Six abilities with scores, modifiers, and saves
Additional Details: Skills, resistances, immunities, gear, senses, languages, CR
Traits: Passive or conditional special abilities
Actions: What the creature does on its turn
Bonus Actions: Quick actions (optional)
Reactions: Triggered responses (optional)
Legendary Actions: For powerful creatures (optional)

Load the full format specification from references/stat-blocks.md when detailed guidance is needed.

Quick Reference Tables

Challenge Rating, XP, and Proficiency Bonus

CR	XP	PB	CR	XP	PB
0	10	+2	6	2,300	+3
1/8	25	+2	7	2,900	+3
1/4	50	+2	8	3,900	+3
1/2	100	+2	9	5,000	+4
1	200	+2	10	5,900	+4
2	450	+2	11	7,200	+4
3	700	+2	12	8,400	+4
4	1,100	+2	13	10,000	+5
5	1,800	+3	14+	See stat-blocks.md	+5 to +9

Hit Dice by Size

Size	Hit Die	Avg HP/Die
Tiny	d4	2.5
Small	d6	3.5
Medium	d8	4.5
Large	d10	5.5
Huge	d12	6.5
Gargantuan	d20	10.5

Creature Types

Aberration, Beast, Celestial, Construct, Dragon, Elemental, Fey, Fiend, Giant, Humanoid, Monstrosity, Ooze, Plant, Undead

Creating a Monster: Step by Step

Step 1: Define the Concept

Determine the creature's narrative role:

What does it look like?
How does it fight (brute, skirmisher, caster, support)?
What makes it interesting or unique?

Step 2: Choose CR and Calculate Base Statistics

Target CR determines the creature's power level. Use these benchmarks:

CR	HP Range	AC	Attack Bonus	Damage/Round	Save DC
1/4	10-20	12-13	+3 to +4	4-5	10-11
1	25-45	13-14	+4 to +5	9-14	12-13
5	80-110	15-16	+6 to +8	35-45	14-15
10	180-220	17-18	+9 to +10	65-75	17-18

Step 3: Assign Ability Scores

Distribute scores based on creature role:

Brute (high STR/CON): 16, 14, 14, 10, 10, 8 Skirmisher (high DEX): 10, 16, 14, 10, 12, 8 Caster (high mental): 8, 14, 14, 16, 12, 10 Balanced: 12, 12, 12, 10, 10, 10

Calculate modifiers: (Score - 10) / 2 rounded down.

Step 4: Determine AC and HP

Armor Class:

Base AC = 10 + DEX modifier
Add armor bonus (light +1, medium +3-5, heavy +6-8)
Add shield (+2)
Natural armor replaces base calculation

Hit Points:

Choose number of Hit Dice based on desired HP
HP = (Hit Dice x average per die) + (Hit Dice x CON modifier)

Step 5: Design Actions

Every creature needs at least one attack. Calculate:

Attack Bonus = Ability modifier + Proficiency Bonus Damage = Dice + Ability modifier

For melee: usually STR-based For ranged: usually DEX-based For finesse weapons: attacker chooses STR or DEX

Step 6: Add Special Abilities

Add traits, bonus actions, or reactions that reinforce the creature concept. Common options:

Pack Tactics: Advantage when ally is adjacent
Nimble Escape: Disengage or Hide as bonus action
Magic Resistance: Advantage on saves vs spells
Multiattack: Multiple attacks per turn
Spellcasting: Access to spells

Step 7: Validate CR

Compare your creature's statistics to the CR benchmarks. Adjust if the offensive or defensive capabilities don't match the target CR.

Using the Templates

NPC Template

Load references/npc-template.md for a complete template with inline comments explaining each field. Copy the template and fill in values for your creature.

Example Stat Block

Load references/npc-example.md for a complete Goblin Warrior stat block with detailed breakdown of how each value was calculated.

Rolling Dice for HP and Damage

Use the mcp__corvran__roll_dice tool for randomized values:

Roll HP for a creature with 3d8 + 6:

Use the mcp__corvran__roll_dice tool:
{ "groups": [{ "n": 3, "d": 8 }], "modifier": 6 }

Roll damage for a longsword attack (1d8 + 3):

Use the mcp__corvran__roll_dice tool:
{ "groups": [{ "n": 1, "d": 8 }], "modifier": 3 }

Roll 2d6 for greatsword damage:

Use the mcp__corvran__roll_dice tool:
{ "groups": [{ "n": 2, "d": 6 }], "modifier": 4 }

The tool returns JSON with the total and individual rolls for narration.

Common Stat Block Patterns

Simple Melee Combatant (CR 1/4)

AC: 13 (leather armor)
HP: 11 (2d8 + 2)
Speed: 30 ft.
STR 14 (+2), DEX 12 (+1), CON 12 (+1)
Attack: +4 to hit, 1d8 + 2 damage

Ranged Attacker (CR 1/2)

AC: 14 (leather armor)
HP: 16 (3d8 + 3)
Speed: 30 ft.
STR 10 (+0), DEX 16 (+3), CON 12 (+1)
Attack: +5 to hit, range 80/320 ft., 1d8 + 3 damage

Tough Brute (CR 2)

AC: 12 (natural armor)
HP: 45 (6d10 + 12)
Speed: 30 ft.
STR 18 (+4), DEX 8 (-1), CON 15 (+2)
Multiattack: Two slam attacks
Attack: +6 to hit, 2d6 + 4 damage

Spellcaster (CR 3)

AC: 12 (15 with mage armor)
HP: 27 (6d8)
Speed: 30 ft.
INT 16 (+3), WIS 12 (+1), CON 10 (+0)
Spellcasting: INT-based, DC 13, +5 to hit
Spells: mage armor, magic missile, fireball (3rd level)

Recording NPCs

After creating a stat block, add it to the adventure's characters.md file for reference during play. Include:

Full stat block
Location where the NPC can be found
Role in the story
Disposition toward the player

License

This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC. The SRD 5.2.1 is licensed under CC-BY-4.0.