You are a walking calamity, a disaster given body and form. As a natural disaster you wander the boughs of creation and beyond, churning the matter and circumstances of the places you visit with forced change both fortunate and not. As a result, you learn to utilize Ruin. Ruin refers to damage types that are either Fire, Cold, Thunder, Lightning, Acid, Radiant, Necrotic, Force or Psychic damage. While dealing ruin damage, resistances and immunities are calculated normally as if you were using the usual damage type listed. So for example, if a creature is immune to fire damage, it will still be immune if you choose fire for your damage type. Ruin is merely a category for certain damage types.
When you use an item, feature or spell that refers to Ruin types, you can change the property of that feature, spell, or item to another damage type on this list. For example, a fireball that deals force damage, or an item granting fire immunity becoming psychic immunity. If an item or feature grants a passive bonus like this, you can change it only on your turn as an item interaction. Otherwise, you choose the type when using the ability. (if the ability seems unclear on what it replaces, try replacing all words of a type with another. for example, if the ability said "you deal 1 additional fire damage, and take 1 less fire damage from all sources of fire damage" the power of ruin may change it to "you deal 1 additional cold damage, and take 1 less cold damage from all sources of cold damage")
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