Spellstrain

Spellstrain is a latent effect in certain places in Archestria. It can affect arcane magic (the Weave) or divine magic (the spirits) or both, depending on the cause. The Weave is generally muted in places where magic has been overused or is suppressed by antimagic. The spirits are generally muted by places where the environment is corrupted, polluted, or weakened.

Spellstrain. When a creature casts a spell using a spell slot, it must expend a second spell slot of the same level or higher, otherwise the spell is lost. The spell is treated as if it were cast at the higher slot level. Fireball costs two 3rd-level slots. If the user expends a 3rd and 4th level slot, it is upcast to 4th level.