Spellwaste

This pool of scintillating slime glistens and glitters like a prismatic liquid.

Source: Monster Encyclopaedia: Ravagers of the Realm Vol. 1 D&D 3.5 by Mongoose Publishing

     Spellwaste is a by-product of a wizards experiment that has gone wrong. This by-product first appears as arcane energy gone wild. The wizard attempts to control this arcane energy before something more sinister goes awry. If the wizard fails in his attempt to control this energy it sometimes will congeal into a slime known as a spellwaste. This arcane slime feeds upon magic and will generally devour the unfortunate mage.

     All Spellwaste require arcane energy to survive. So once that pesky wizard is eaten they will often die off. If an abundance of food is readily available the Spellwaste will continue to survive. Either by draining the environment which is rich in magic or finding other practitioners to consume. A Spellwaste will thrive in a place of magic or maybe an arcane academy. A single cantrip can feed a Spellwaste for days while spells of higher level is like a feast that can sustain them for weeks.

     There are even some wizards who keep the Spellwaste locked away in a dungeon and feed them their leftover magical experiments.

Combat: The Spellwaste is a Large Ooze and a Challenge Level 5 creature. Their main tactic in combat involves making a straight run at a wizard or a person holding an object of magic. These mindless oozes always goes for the juiciest prey.

Spelldrain: If a Spellwaste slam attack hits in combat it automatically drains magic from the victim. Magic drained is done in the following order:
1. If the target has any spells currently active on him, it drains the highest level spell.
2. If the target has any prepared spells, it drains the highest level spell prepared.
3. If the target has any spontaneous spells, it drains the highest level spell slot left unused.
The Spellwaste Absorb Magic ability is automatically activated by Spelldrain

Absorb Magic: Any spell that targets the Spellwaste or absorbed through Spelldrain heals this arcane ooze 1d6 points per level of the spell. An area radius spell that catches the Spellwaste heals it 1 point per level of the spell. Otherwise it has no effect on it. Spells have no effect on the Spellwaste except for healing it.

Scent Magic: Can automatically detect any magic or spellcaster within a 60ft radius

-Misc Abilities:  Ooze type immunities, Damage Reduction 10/slashing