These monsters are simple and horrible. Necrodwarves create them by feeding regular leeches on the undead remains they have animated. After a few generations, the “beardleeches” are ready for service.
A beardleech can drain 1 hit point a minute from a living, sentient target—anything that would serve as a sacrifice for the necrodwarf. The beardleech can hold up to 4 hit points, growing from something the size of a pinky finger to something the size of a banana.
A necrodwarf can devour a beardleech in 1 round. (The details of how this looks, sounds, smells, etc.—that’s up to the DM…) The necrodwarf gains 1 hit point per hit point stored in the beardleech.
Beardleeches are so named because some necrodwarves will hang them from armor behind the protection of the beard, out of sight and ready for quick consumption.
Even necrodwarves have limits to what they can bear. A necrodwarf can only consume 1 beardleech per hit die/level in a day.
Trying to eat a beardleech does not help a character or monster who has not been printed with runic necromancy.