Arijani

Limited Magic Immunity. The rakshasa can't be affected or detected by spells of 6th level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects.

Dark Lord’s Bane. Arijani cannot be harmed until first pierced with a weapon made from Shar’s Bane.

Dark Lord’s Foe. attack rolls of a natural 18, 19, or 20, when made against Arijani in the presence of his chosen
enemy, count as critical hits.

Devil’s Sight. Arijani can see in magical darkness.
Eldritch Master (1/Day). Arijani can regain all of his spell slots by concentrating uninterruptedly for one minute.
Descent through the Dark (3/Day). When Arijani hits a creature with an attack that requires an attack roll, he can use this feature to transport the target to a devastating pocket dimension of pure darkness and loss, which represents Shar’s dominion. At the end of Arijani’s next turn, the target returns to the space it previously occupied, or the nearest unoccupied space. If the target is not a fiend, it takes 10d10 psychic
damage as it reels from its horrific experience.
Master of Illusion. Creatures that Arijani targets with spells from the Illusion school suffer disadvantage on
their saving throws.
Legendary Resistance (3/Day). If Arijani fails a saving throw, he can choose to succeed instead.

Innate Spellcasting. The rakshasa's innate spellcasting ability is Charisma (spell save DC 26, +18 to hit with spell attacks). He can cast alter self, detect thoughts, and major illusion at will. He can cast phantasmal killer, mirage arcane, and misty step three times per day each without expending a spell slot, and weird once per day without expending a spell slot.

Arijani has the following spells prepared:

Cantrips (at will): [spel]dancing lights[/spell],eldritch blast (4 beams), fire bolt, mage hand
1st—6th level (4 5th-level slots): burning hands, calm emotions, command, confusion, darkness,
fireball, fire shield, flame strike, hallow, hallucinatory terrain, hypnotic pattern, invisibility, Mordenkainen’s private sanctum, Nystul’s magic aura, phantasmal force, phantasmal killer, silent image, scorching ray, telekinesis,
6th level (2 slots): mental prison, programmed illusion
7th level (2 slots): mirage arcane,project image, simulacrum, teleport
8th level (2 slots): dominate monster, glibness, power word stun
9th level (2 slots): weird, time stop

Multiattack. The rakshasa makes two claw attacks.

Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 9 (2d6 + 2) slashing damage, and the target is cursed if it is a creature. The magical curse takes effect whenever the target takes a short or long rest, filling the target's thoughts with horrible images and dreams. The cursed target gains no benefit from finishing a short or long rest. The curse lasts until it is lifted by a remove curse spell or similar magic.

Consume Magic. Arijani expends a spell slot to gain 1d4 x that spell slot’s level in hit points.

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

Attack. Arijani makes one claw attack.
Cantrip (Costs 2 Actions). Arijani casts a cantrip.
Transfixing Sympathies (Costs 3 Actions). All PCs attempt a DC 20 Charisma saving throw. The Dungeon Master chooses any two who fail; their players must exchange character sheets until Arijani’s next turn.
Move. Arijani moves up to his speed without provoking attacks of opportunity.