Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a huge orcus. The swarm can't regain hit points or gain temporary hit points.

Wand of Orcus. The wand has 70 charges, and any of its properties that require a saving throw have a save DC of 18. While holding it, the swarm can use an action to cast animate dead, blight, or speak with dead. Alternatively, they can expend 1 or more of the wand's charges to cast one of the following spells from it: circle of death (1 charge), finger of death (1 charge), or power word kill (2 charges). The wand regains ld4 + 3 charges daily at dawn.

While holding the wand, the swarm can use an action to conjure undead creatures whose combined average hit points don't exceed 5000. These undead magically rise up from the ground or otherwise form in unoccupied spaces within 400 feet of the swarm and obey there commands until they are destroyed or until they dismisses them as an action. Once this property of the wand is used, the property can't be used again until the next dawn.

Innate Spellcasting. The swarms spellcasting ability is Charisma (spell save DC 23, +15 to hit with spell attacks). they can innately cast the following spells, requiring no material components:

At will: chill touch (17th level), detect magic

30/day each: create undead, dispel magic

10/day: time stop

Legendary Resistance (3/Day). If the swarm fails a saving throw, they can choose to succeed instead.

Magic Resistance. The swarm has advantage on saving throws against spells and other magical effects.

Magic Weapons. The swarms weapon attacks are magical.

Master of Undeath. When the swarm casts animate dead or create undead, they chooses the level at which the spell is cast, and the creatures created by the spells remain under there control indefinitely. Additionally, they can cast create undead even when it isn't night.

Multiattack. Orcus makes twenty Wand of Orcus attacks, or make ten tails attacks

Wand of Orcus. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 21 (3d8 + 8) bludgeoning damage plus 13 (2d12) necrotic damage.

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

Orcus can take 30 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.

Tail. Orcus makes one tail attack.

A Taste of Undeath. Orcus casts chill touch (17th level).

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.