Creature types and damage

Creature types are entity type tags, #fundamentum:creature/<type>. Fundamentum starts with undead, arthropod, aquatic, illager, construct and boss; any mod or datapack may add types or members. For example, data/fundamentum/tags/entity_type/creature/construct.json:

{"values": ["minecraft:iron_golem", "minecraft:snow_golem", "mymod:clockwork_knight"]}

Damage rules

Rules scale damage against a type. File: data/<ns>/fundamentum/creature_damage/<name>.json

{"creature": "undead", "weapon": "#mymod:holy_weapons", "damage": "#minecraft:is_fire", "multiplier": 1.5}

Field	Required	Meaning
creature	yes	the type (undead, or a full tag id)
multiplier	yes	damage multiplier
weapon	no	an item tag the weapon must be in
damage	no	a damage type tag the hit must be in

A rule with neither weapon nor damage applies to every hit on that type. Matching rules multiply together. Ostentum uses one to make pickaxes hit constructs for 1.5x.