Arcane hunger

When you hit a spellcaster with any attack that causes damage, there is a 50% chance (1d100) that instead of HP damage, they will instead drain magic from the target. This will deplete 1 spell slot level for every 10hp damage that would otherwise have been caused (rounded up). eg: 27hp damage would be 3 spell slot levels.  (this could be 3xlvl 1 slots, 1xlvl 3 slot, 1xlvl1 + 1xlvl2 slots).

Optional (DM discretion), these drained slots replenish any used slots for the character.