Contagion Gaze. When a humanoid that can see Xancrown’s eyes starts its turn within 30 feet of the demon lord, Xancrown can force it to make a DC 24 Constitution saving throw if Xancrown isn’t incapacitated and can see the humanoid. On a failure the humanoid is afflicted with the demonplague.
Unless surprised, a humanoid can avert its eyes to avoid the saving throw at the start of its turn. If the humanoid does so, it can’t see Xancrown until the start of its next turn, when it can avert its eyes again. If the humanoid looks at Xancrown in the meantime, it must immediately make the save.

Death Slaves. When a creature infected with demonplague is killed by Xancrown, it rises on its next turn as a valok (page 43) under his control.

Spellcasting. Xancrown is an innate spellcaster. Its spellcasting ability is Charisma (spell save DC 23, +15 to hit with spell attacks). Xancrown can innately cast the following spells:
Cantrips (at will): chill touch (17th level), ray of sickness

3/day each: counterspell, blight

1/day: summon Glabrezu General 

Legendary Resistance (3/day). If Xancrown fails a saving throw, he can choose to succeed instead.

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

Magic Weapons. Xancrown's weapon attacks are magical.

Multiattack. Xancrown makes 4 attacks: one with his bite, one with his tail, and two with his claws.

Bite. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 27 (3d12 + 8) piercing damage. When Xancrown bites a creature that has at least one head and rolls a 20 on the attack roll, it takes an extra 27 (6d8) piercing damage from the hit.

Claw. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 21 (3d8 + 4) slashing damage and the target is grappled (escape DC 24). Until this grapple ends, the target is restrained and Xancrown can’t use this claw to attack another target. Xancrown can grapple up to two creatures this way at once. 

Tail. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 21 (3d8 + 8) piercing damage and the target must make a DC 24 Constitution saving throw or become poisoned for 1 minute. A creature poisoned this way can repeat the saving throw at the end of its turn, ending the poisoned condition early on a success

Throw. Ranged Weapon Attack: +19 to hit, range 100 ft., one target. If the creature hits a wall before landing, it takes Hit: 21 (5d8 + 8) bludgeoning damage. If the creature hits another creature, that creature takes the damage instead.

Enter the description for how bonus actions work for your monster here.

Wrath of the Plague God. 13(3d8) necrotic damage to an adjacent foe who hits Xancrown with an attack.

Strength of the Plague God. If Xancrown has a creature [condition]grappled[/grappled] in his claws, can use it's reaction to throw that creature.

Xancrown 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. Xancrown regains spent legendary actions at the start of his turn:

Tail (Costs 1 Actions). Xancrown makes one Tail attack.

Cast Spell(Costs 1 Actions). Xancrown casts chill touch (17th level), ray of sickness(3rd level), blight

Weakening Aura (Costs 2 Actions). Xancrown sends out waves of disease from his body. Each creature within 20 feet of him must succeed on a DC 24 constitution saving throw or gain a level of exhaustion.

When Xancrown reaches 75% health, he regains all legendary actions for that round and casts negative energy flood.

When Xancrown reaches 50% health, he regains all legendary actions for that round and casts circle of death.

When Xancrown reaches 10% health, he regains all legendary actions for that round and instantly animates all creatures that have fallen within the last 24 hours. Any mirror images now take the place of dear loved ones instead of images of themselves.