Arcanoplasm

Why wizards always think nothing bad will come of their meddling is really a good puzzle.

Arcanoplasms are spawned from failed magic experiment or the friction between places of power, the result of pollution and clashing arcane forces.

Description

Shimmering with countless colors across its protoplasmic bulk, Arcanoplasms are amobea-like creatures made of magical discharge. Bursts of energy pop from their bodies every now and then like bubbles breaking water, while spells cast at the creature hangs suspended within them as if in stasis. They feed on magic of all kinds and are drawn to Demesnes or other sites of power, but are happy to devour any hapless spellcaster they come across.

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Although they are mindless predators, Arcanoplasms have an uncanny ability to sense and seek out magic of all kinds. If confronted, it will always target any spellcasters first, and any magic items second, seeking to draw them into itself to leech the magic out of its victim. These usually drown in the process, at which point the Arcanoplasm leaves their victim to seek their next meal. Most Plasms are the size of a horse, with their size depending on how much they've been able to feed.
Such victims, whether living or artifice, are slowly drained of their magic over a period of days. For spellcaster, the process is exhausting and potentially fatal, even without the threat of suffocation.

Magic items take longer, and some items are too powerful to be devoured - in such cases, the Arcanoplasm will continue to try and feed until it explodes from an overload of energy. Spells cast against the Arcanoplasm are consumed on impact, absorbing into their bulk and visible from within and being devoured.
While hunting, the Arcanoplasms will unleash these spells at random, ejecting fireballs and magic missiles with wild abandon in search of a juicier morsel.

This seeming invulnerability to magic make them particularly feared by spellcasters, but some have found that if they overload an Arcanoplasms ability to absorb magic, they'll burst open in much the same fashion as if trying to devour an artifact.

by http://www.tribalartasia.com/

Spell-Colored

Arcanoplasms change appearance and sometimes abilities depending on the magic they have consumed. A plasm well-fed on Earth magic might become slow but hard, sporting a shroud of metallic pebbles on its blubbery surface, or become hot to the touch when feasting on Fire magic.

by Rock Cafe.info

In the World

Arcanoplasms are still uncommon in Ithekshem, spawning infrequently from meddling wizards or magical friction in Demesnes and Leylines. Impervious against all attempts of magical control as they just eat the spell, they have so far resisted most attempts by wizards to use them against their rivals. More cunning wizards forgo control altogether and just load any captured Arcanoplasms up into a catapult and launch them into the homes and manses of their enemies, but the most cunning wizards all do whatever they can to stay well away from Arcanoplasms. They're a frequent target for paid adventurers to go exterminate, with particularly fearsome examples needing to be captured instead as they hold some sort of one-of-a-kind spell within their bodies.

Needless to say, such trips usually never end the way anybody involved expects them to.

Others have attempted to use the Arcanoplasms as storage for spells to be unleashed later, but have yet managed to keep the plasms from devouring the spell until they fizzle out.