A small fraction of closets should contain a skeleton.

To help differentiate bone devils from being boring popcorn monsters with no special attacks or abilities, give them the ability to summon skeletons.

This should come with reasonable limits, such as setting their mspec_used field so that they can only do it every so often, and preventing them from doing it at all if there are a certain number of skeletons around them or on the whole level. Possibly limit it so that if they are not within the “misc item use” distance from the player, they will limit themselves to creating 0 or 1 skeletons.

Spell of necromancy that turns a corpse you’re standing on (or possibly adjacent to you) into a tame undead. Probably in the clerical school, though a case could be made for matter (and certain variants have a school of necromancy which this would much more directly fit into). It will work on corpses of undead that you have killed. Casting this spell would have some sort of penalty for lawfuls; there would be some sort of warning before trying to cast it.

You can normally only create zombies or mummies when an undead monster exists for the kind of corpse you are animating (i.e. a jackal can’t be turned into undead but a gnome can turn into a gnome zombie/mummy). For intelligent monsters that have no counterpart, the spell can create ghosts.

At a high enough skill or caster level, you can create skeletons from the corpse of any vertebrate, and maybe even vampires from humanoids. You can also create wraiths and shades from monsters that have no undead counterparts.

Skeletons may drop skeleton keys when killed.

Skeletons can spawn in places that aren’t Orcus-town.

The Staff of Necromancy: an artifact quarterstaff that deals double damage to non-drain-resistant creatures (but doesn’t life-drain them as the Staff of Aesculapius does), grants cold resistance and curse resistance when wielded, and can be invoked to raise one vertebrate corpse on your square as a tame skeleton.

Cursed amulets of life saving allow you to remain in the game, at the cost of bringing you back as a skeleton.

TDTTOE: Skeletons use gendered pronouns if and only if the player is an Archeologist (and therefore used to analyzing bones).

Skeletons, or any monster with the MS_BONES sound, rattle their bones periodically, which frighten the player into helplessness for a few turns.