Unelem

Unelem are a form of ritual undead that heavily integrate elemental powers in the casting.  Unelem are created by either ritually infusing undead with incredible elemental power, or creating an undead with a strong elemental core.  The elemental powers are locked in an entropic stasis, constantly decaying and drawing in more power.  The strongest of these are cast with opposing elementals with the intent of combining to create the catalyst that would expedite the elemental entropy.  Though these aberrant forms rarely survive for very long.   Most however, remain cast using one element, and thus within the control of the caster.
Notable Unelem include Brimstone Walkers, akin to zombies with molten lava-like skin that sloughs off as they move about, and hardening to an impenetrable shell when they stop moving.