Rakshasa

The monster automatically succeeds on saving throws against spells and other magical effects, and the attack rolls of spells automatically miss it. Without the monster's permission, no spell can observe the monster remotely or detect its thoughts, creature type, or alignment.

If the monster dies outside Gehenna, its body dissolves into ichor, and it gains a new body instantly and revives with all its Hit Points in Gehenna.

The monster makes three attacks.

. . If the target is a creature, it is cursed. While cursed, the target gains no benefit from finishing a Short or Long Rest.

Wisdom Saving Throw: DC , each in a -foot Emanation originating from the monster.
Failure: damage, and the target has the Frightened and Incapacitated conditions until the start of the monster's next turn.

The monster casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC ):
At Will: Detect Magic, Detect Thoughts, Disguise Self, Mage Hand, Minor Illusion
1/Day Each: Fly, Invisibility, Major Image, Plane Shift