Orcus
Orcus is the Demon Lord of the 113th Layer of the Abyss, known as The Belly of Death. He has been called the Prince of the Undead, and is infamous for sitting on his throne while minions, cultists, and lesser agents do his bidding.
In all of Orcus's incarnations, it/he/whatever appears as the goat-headed, cloven-hooved monster from medieval depictions of The Devil, and wields a rod or mace dubbed "the Wand of Orcus," and has a fascination with or domain over the undead. He has been featured as the BBEG of multiple adventures, campaigns, and campaign settings, most prominently in Forgotten Realms via Dead Gods and least prominently in Ghostwalk, where he plots to kill all ghosts and living beings in the setting. Most of the time, he spends the entire campaign sitting on his throne in the Abyss, while the players work their way through the machinations of his minions and worshippers. Whenever he does something awesomely evil, it happens off-screen. Once, he even showed up to warn the player characters from looting his stuff before returning to The Abyss, preferring to leave a verbal smackdown rather than get his hands dirty. This would lead to his death.
The only D&D edition that didn't have Orcus was AD&D 2nd Ed, mostly because of a certain someone who wanted to get rid of anything un-Christian and anything else she didn't like about D&D (which was anything that wasn't her beloved Buck Rogers... I wish I was joking). Orcus did come back in 2E with Planescape supplement as the secret BBEG behind the Dead Gods adventure plotline.
the following is copypasta
3.5E Orcus
The massive, bloated demon stands 15 feet tall, his immense frame a hideous combination of muscle and bloated flesh. Its head is that of a ram with a great maw filled with tusks, and its thick-furred legs end in cloven hooves. Leathery wings and a barb-tipped tail complete the picture of the archetypal demon.
- Orcus, Prince of the Undead [CR 22] CE Large outsider (chaotic, evil, extraplanar, tanar’ri)
- Init+10 ; Senses darkvision 60 ft., true seeing; Listen +34, Spot +34
- Languages Abyssal, Common; telepathy 300 ft.
- AC 48, touch 20, flat-footed 42 (–1 size, +6 Dex, +28 natural, +5 deflection)
- hp 455 (26 HD); DR 20/cold iron and good
- Immunities: ability drain, critical hits, death effects, electricity, energy drain, mind-affecting spells and abilities, negative energy, paralysis, poison, sneak attacks
- Resistances: acid 10, cold 10, fire 10; SR35
- Saves: Fort+28 , Ref+21, Will+20
- Speed: 20 ft. (4 squares), fly 40 ft. (average)
- Melee: Wand of Orcus +44/+39/+34/+29 (2d6+18/19–20 plus 2d6 unholy plus 2d6 chaotic plus death) and claw +35 (1d6+6) and gore +35 (1d8+6) and sting +35 (1d6+6 plus poison)
- Space 10 ft.; Reach 10 ft.
- Base Atk+26 ; Grp+ 42
- Special Actions summon tanar’ri, summon undead
- Spell-Like Abilities (CL 20th): At will: astral projection, command undead (DC 18), desecrate, detect good, detect law, enervation(+31 ranged touch), greater dispel magic, greater teleport, telekinesis (DC 19), unhallow, unholy blight (DC 18); 3/day: quickened enervation (+31 ranged touch), finger of death (DC 23), symbol of death (DC 24); 1/day: wail of the banshee (DC 25)
- Abilities: Str 35, Dex 23, Con 37, Int 27, Wis 20, Cha 19
- Special Qualities: tanar’ri traits (see Fiendish Codex I, page 28)
- Feats: Dark Speech*, Greater Spell Focus (necromancy), Hover, Improved Critical (heavy mace), Improved Initiative, Multiattack, Quicken Spell-Like Ability (enervation), Spell Focus (necromancy), Weapon Focus (heavy mace)
- Skills: Bluff +33, Climb +41, Concentration +42, Craft (alchemy) +37, Diplomacy +8, Escape Artist +35, Forgery +37, Intimidate +35, Knowledge (arcana) +37, Knowledge (religion) +37, Knowledge (the planes) +37, Listen +34, Search +37, Sense Motive +34, Spellcraft +39, Spot +34, Use Magic Device +33
4E Avatar of Orcus
- Aspect of Orcus - Level 24 Elite Brute
- Large elemental humanoid (demon)
- Initiative +15 Perception +21 (darkvision)
- Aura: Lesser Aura of Death (Necrotic) aura 10 inflicts necrotic 5
- HP 560; AC 37; Fortitude 39, Reflex 35, Will 36
- Immune: disease, poison
- Resists: 20 necrotic, 10 variable (may change 3/encounter as minor action)
- Saving Throws +2
- Speed 6, fly 8 (clumsy)
- Action Points 1
- Melee attack(default): Skull Mace (standard; at-will) (Necrotic, Weapon) Reach 2; +27 vs. AC; 2d10 + 5 damage, and the target is weakened (save ends).
- Melee attack: Tail Lash (immediate reaction, when an enemy moves or shifts into a square adjacent to the aspect of Orcus; at-will) +27 vs. AC; 2d6 + 10 damage, and the target is knocked prone.
- Close Burst: Necrotic Burst (standard; at-will) (Necrotic) Close burst 3; targets enemies; +23 vs. Fortitude; 1d10 + 5 necrotic damage.
- Alignment Chaotic evil Languages Abyssal, Common
- Skills Arcana +23, History +23, Intimidate +24, Religion +23
- Str 30 (+22) Dex 17 (+15) Wis 19 (+16) Con 30 (+22) Int 22 (+18) Cha 25 (+19)
SPOILERS
Tenebrous was a deity of undeath that was actually an undead Orcus. Yes, we know, yo dawg and all that. He ends up slaying several minor gods of nonhuman races and is defeated by the party in Dead Gods. However, faithful demon worshippers bring him back from the dead, allowing him to exist as an enemy in 4e Forgotten Realms.
Tenebrous remains as a Vestige capable of being bound to Binder player characters in 3.5e. Presumably he can also be used by Pact Wizards in 4e.