Laraken

A laraken, or magic-drainer, is a glowing yellow sphere with two yellow tentacles on opposite sides. It can blink out of sight and reappear elsewhere an instant later. Distantly related to another magic-devouring mist creature, it must keep its body wet and so stays in swamps.

Hating all intelligent life, larakens attack any party with magic or spellcasters but will not pursue onto dry land. A tentacle lashes once per round for 1d4+1 and drains a spell from a caster or a charge or plus from an item; a weapon striking a tentacle is drained too. The laraken gains what it steals: a sword's plus becomes +1 to its own attacks and damage, and a stolen wizard spell can be cast by it. It absorbs spells aimed at it as well.

It drains memorized spells first, then charges from items that produce spell effects, then pluses, always taking a caster's highest-level spell (random among equals). The drained caster must make a Wisdom-adjusted save vs. death magic or fall unconscious for 1d10 turns.

A tentacle is severed after taking 10% of the creature's total hit points. Regrowing it costs one absorbed spell level per hit point of tentacle, and levels used this way give no other benefit. Three spell levels can also heal 1 hp.

A great conjurer summoned larakens from a soaking-wet alternate Prime Material world. Below 95% humidity they are uncomfortable, so these southern lands seem dry to them. His gate to the Water plane floods his swamp with enchanted water, giving them abundant food.

Larakens live on magic alone, needing three spell levels daily, and attack out of hatred for their summoner, extended to anything vaguely humanoid. They reproduce here only by absorbing a regenerate spell, which grows a new laraken from a cut tentacle, so they are fortunately rare.