That's where Furfur comes in. The Prince of Secrets is something I've had in my head for a while now, but two recent events pushed me to finally write him up. First, Zak noted the creepiness of stag-headed monsters (I agree). Second, Will posted a link to a PDF of the famed Infernal Dictionary, which, to my mind, contains some of the most disturbing images of demons I've ever come across. I've been looking for a reproduction of it online for a while now, so I'm very grateful to Will for finally bringing one to my attention.
The text in the quote box below is hereby designated Open Game Content via the Open Game License.
Furfur (Demon Lord)
No Enc.: Unique
Movement: 120' (40'); Fly: 240' (80')
Armor Class: -6
Hit Dice: 120 hp (20 HD)
Attacks: 2 (claws)
Hoard Class: IV, V, VIII, IX
The demon lord Furfur (also known as Furcifer and the Prince of Secrets) is well known among sages and seekers after forbidden knowledge, for that is his area of influence. He appears as a creature with the hindquarters and head of a stag, but the breast and arms of a man, and bat-like wings that carry him through the air. His tail bursts into flames when Furfur is angry or displeased, something that does not occur very often, as Furfur is generally described as an eerily calm and rational demon lord, more interested in conversation than combat.
If pressed, however, this demon lord is more than capable of defending himself. His own physical attacks are somewhat weak but he commands a variety of magical abilities: astral projection, call lightning, charm person, clairvoyance, clairaudience, continual darkness, control weather (greater), control winds, detect invisibility, detect magic, ESP, legend lore, read languages, read magic, shape change, suggestion, unholy word, wind walk. These abilities are usable at will. Once per day, Furfur can use feeblemind, power word (any), and symbol (any). He is only affected by +2 or better weapons.
The demon lord may gate (80% probability of success) a glabrezu, hezrou, marilith, nalfeshnee, or vrock demon. Furfur may also summon other creatures as a monster summoning spell. He may bring forth 4d4 harpies, 3d4 perytons, or 2d4 gargoyles, or 1d4 succubi.
Furfur is reputed to know the answer to any question, including those even the gods have. In exchange for a promise of eternal servitude after death, the Prince of Secrets will answer any three questions posed to him with absolute and total truth -- although many come to regret Furfur's candor even before they must fulfill their end of the bargain.