Cisum Homebrew, Monsters of the Multivers
The Thin White Duke casts one of the following spells, using Charisma as the spellcasting ability (spell save DC 15):
The Devil's Own Luck (Recharges after a Short or Long Rest). When the Duke makes an ability check or saving throw, he can add a d10 to the roll. He can do this after the roll is made but before any of the roll's effects occur.
Legendary Resistance (2/day). If the Thin White Duke fails a saving throw, he can choose to succeed instead.
Multiattack. The Duke makes three Scimitar attacks.
Scimitar. Melee Weapon Attack. +6 to hit, reach 5ft., one target. Hit: 6 (1d6+3) 1d6+3 bludgeoning damage plus 14 (4d6) 4d6 fire damage.
Hellfire. Reddish-purple flame explodes in a 10-foot-sphere centered on a point within 120 feet of the Duke. Each creature in that area must make a DC 15 Dexterity saving throw, taking 16 (3d10) fire damage and 11 (2d10) necrotic damage on a failed save, or half as much damage on a successful one.
Shadow Man. As a bonus action, if the Duke is standing in dim light or darkness, he can teleport up to 60 feet to an unoccupied space he can see that is also dim light or darkness. He gets advantage on the first melee attack made before the end of the turn.
Fiendish Rebuke (3/Day). In response to being damaged by a visible creature within 60 feet of him, the Duke forces that creature to make a DC 15 Constitution saving throw, taking 22 (4d10) necrotic damage on a failed save, or half as much on a successful one.
The Thin White Duke 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. The Duke regains spent legendary actions at the start of his turn.
Ground Control to Major Tom. The Duke can pass a concentration spell to an ally within 20 feet. If the chosen ally is a spellcaster, it has advantage on concentration checks. Only one concentration spell may be passed a time. If the Duke uses another instance of this while an ally is maintaining a concentration spell, the first passed spell ends.The Duke's usually casts fire shield or mage armor at the start of a battle he feels will need his personal involvement. He is content to stand back and use Hellfire to do damage most of the time. While out of melee range, he often uses command to throw off his enemies or prestidigitation to try and disrupt casters using concentration spells. If an enemy becomes rather annoying to him personally, he will use banishment to remove them and allow his personal guard to focus on other targets. If an enemy becomes particularly problematic, the Duke will cast plane shift to remove them from the equasion all together. The Duke rarely resorts to melee attacks unless the target seems close to defeat or lacking in armor.
The Thin White Duke is the head of state for the Blackstar Malkuth, the nation hailing from the Major Keys. He is most often found in his homeland busying himself with running his domain and his constant plans of conquest. The Thin White Duke can be used as the BBEG for the war time campaign where the Blackstar Malkuth is trying to invade and conquer the Acappellago. As the third act boss fight, he can be found aboard his flagship sailing with a fleet to discuss an alliance with mob boss Louie Louie on Panama.