There are a number of unusual undead in Vandalmál, including several varieties of skeletons and zombies that are stronger and more crafty than the more common types. Erudite and wicked necromancers and blood sorcerers truck in creating these creatures, although they do occasionally arise "naturally" as a result of particular powerful emanations from the Negative Energy plane.
Knowledgeable practitioners can make modifications to the animate dead spell to create more powerful versions of skeletons and zombies - special components must be added alongside those required for the spell normally, and occasionally the spell must be cast using a higher level spell slot as well.