Seetangeist

Seetangeists are composed of the bloated corpses of marine life, encrusted with bleached coral and wrapped in rotting seaweed. Although occasionally created by water-breathing necromancers, seetangeists most often arise when an aquatic ecosystem collapses, resulting in the deaths of countless creatures. Bound by spiritual trauma, these corpses merge together, forming a writhing school of flesh and decaying plant matter that seeks out and consumes what life remains in the region.

The disasters that lead to the creation of seetangeists are typically caused by mortal interference: overhunting, ecological overexploitation, pollution, or necromantic magic and experimentation.

When disaster befalls the natural world, a different kind of undead may rise, entirely unlike risen humanoids. Most plants, fish, insects, and other animals don't possess a psychology complex enough to experience the emotional catharsis required to return as undead without intervention from an outside force. Mass extinction can serve as that outside force.

Humanoids clear-cutting an entire forest or a seaside community wiping out a species critical to their ecosystem can spawn a verdorite. The collective spiritual energy and emotional distress of a dying biome powers these undead, formed as conglomerates or representatives.