This is in one of the old d&d books , not sure which one, maybe BOVD..
They're zombies with a spell stored in them... some are programmed to use it on the first enemy they see..
others are programmed so that when the zombie is cut open (destroyed) the spell fires off at the zombie.
Cloud spells and area of effect spells are the ones that are usually put in the "detonator" zombies.
The zombies that fire off their spell at the first target usually have a RAY spell in them, once the spell has been used it's gone. the zombie can only store a spell of a level that the zombie could cast if it was alive.
So a level 3 zombie could store 1 level 1 spell, or 1 level 2 spell.
level 7 zombies could store a spell of level 1,2,3 or 4
Some of these zombies can have a "see enemy" spell and a detonator spell, those are the most dangerous and difficult to make.
More often than not the spells stored are necromancy, but evocation and conjuration is common too.
Zombie with grease spells inside them are a riot.