Magebane -- A poison to keep those pesky mages in line

[PhoenixPhyre]

Magebane (injury or ingested poison)

This oily poison is made from the sap of burnroot vines, treated with an alchemical concoction made from the blood of a spellcaster. When a target is exposed to this poison, they must make a DC 14 Constitution saving throw or be poisoned for 1 hour. Whenever a target poisoned by magebane expends a spell slot or casts a spell (even if no spell slot is expended in doing so), they take 2d6 poison damage per spell level, with cantrips counting as spell level 1/2 (ie incurring 1d6 poison damage).