Barrier wights are species of undead (largely unrelated to normal wights) created by a specific necromantic process that inflates the corpse full of contagious pus and poisonous gas, ready to burst. Barrier wights are then given one instruction: move toward invaders, and, when you are close, destroy yourself.
The act of damaging a barrier wight causes one to violently explode in a shower of pus and gas that paralyzes the victim, and infects them with a dreadful disease wherein they loose half their health each day until cured.
Barrier wights will often be placed in narrow locations where their explosion is difficult to avoid, or in massive groups, where their explosions can trigger others to explode in a catastrophic cascade of pus and gas.
Occasionally, a semi-active chunk of the barrier wight about the size of a grapefruit will remain, instilled with enough necromantic energy to burst if thrown roughly or pierced. The chunk deals some explosive damage, and infects a target with the disease. Necromancers seek out these small necromantic bombs for their personal and experimental use.