Special Equipment.

Orcus wields the Wand of Orcus.

Legendary Resistance (3/Day).

If Orcus fails a saving throw, he can choose to succeed instead.

Magic Resistance.

Orcus has advantage on saving throws against spells and other magical effects.

Master of Undeath.

Orcus can cast animate dead (at will) and create undead (3/day). He chooses the level at which the spells are cast, and the creatures created by them remain under his control indefinitely. Additionally, he can cast create undead even when it isn't night.

Actions

Multiattack.

Orcus makes three Wand of Orcus, Tail, or Necrotic Bolt attacks.

Wand of Orcus.

Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 24 (3d8 + 11) bludgeoning damage plus 13 (2d12) necrotic damage.

Tail.

Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 21 (3d8 + 8) force damage plus 9 (2d8) poison damage.

Necrotic Bolt.

Ranged Spell Attack: +15 to hit, range 120 ft., one target. Hit: 29 (5d8 + 7) necrotic damage.

Conjure Undead (1/Day).

While holding the Wand of Orcus, Orcus conjures Undead creatures whose combined average hit points don't exceed 500. These creatures magically rise up from the ground or otherwise form in unoccupied spaces within 300 feet of Orcus and obey his commands until they are destroyed or until he dismisses them as an action.

Spellcasting.

Orcus casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 23):

At will: detect magic
3/day: dispel magic
1/day: time stop

Wand Spellcasting.

While holding the Wand of Orcus, Orcus casts one of the following spells (spell save DC 18), some of which require charges; the wand has 7 charges to fuel these spells, and it regains 1d4 + 3 charges daily at dawn:

At will: animate dead (as an action), blight, speak with dead
2 charges: power word kill
1 charge each: circle of death, finger of death

Legendary Actions

Orcus can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Orcus regains spent legendary actions at the start of his turn.

Attack. Orcus makes one Tail or Necrotic Bolt attack.
Creeping Death (Costs 2 Actions). Orcus chooses a point on the ground that he can see within 100 feet of him. A cylinder of swirling necrotic energy 60 feet tall and with a 10-foot radius rises from that point and lasts until the end of Orcus's next turn. Creatures in that area have vulnerability to necrotic damage.

Lair Actions

On Initiative count 20 (losing initiative ties), Orcus can take a lair action to cause one of the following effects; he can't use the same effect two rounds in a row:

Deadly Utterance. Orcus's voice booms throughout the lair. His utterance causes one creature of his choice to be subjected to power word kill. Orcus needn't see the creature, but he must be aware that the individual is in the lair.
Grasp of the Dead. Orcus causes skeletal arms to rise from an area on the ground in a 20-foot square that he can see. They last until the next initiative count 20. Each creature in that area when the arms appear must succeed on a DC 23 Strength saving throw or be restrained until the arms disappear or until Orcus releases them (no action required).
Undead Servants. Orcus causes up to six corpses within the lair to rise as skeletons, zombies, or ghouls. These undead obey his telepathic commands, which can reach anywhere in the lair.

Regional Effects

The region containing Orcus's lair is warped by his magic, creating one or more of the following effects:

Charnel Realm. The air is filled with the stench of rotting flesh, and buzzing flies grow thick within the region.
Undead Beasts. Dead Beasts periodically animate as Undead mockeries of their former selves. Skeletal and zombie versions of local wildlife are commonly seen in the area.

If Orcus dies, these effects fade over the course of 1d10 days.