Actions

Multiattack.
The rakshasa makes two claw attacks, it can replace one of the claw attacks with casting a spell or replace both and use its Flames of Surtr ability.

Claw.
Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 9 (2d6 + 2) slashing damage + 18 (4d8) fire 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.

Flames Of Surtr (RECHARGE 5-6):

Fire Cone - The orb eminates fire in a 30-foot cone. Each creature in that area must make a DC 16 Dexterity saving throw, taking 14d6 fire damage on a failed save, or half as much damage on a successful one.
Lava Beam - The orb eminates a strong conentrated beam of lava in a 40ft long, 5ft wide line. Each creature in that area must make a DC 14 Dexterity saving throw, taking 12d8 fire damage on a failed save, or half as much damage on a successful one.
 

Boss Actions

These actions can be used for free after any character's turn.

Round 1: The Heart of Surtr summons 2 fire elementals on an unocupied space within 60ft.

Round 2: The rakshasa casts Charn Person on 5 targets. This does not count against the number of times it can cast the spell.

Round 3 (ult): The rakshasa recharges and immediately uses its Flames Of Surtr attack

Round 4: The rakshasa shifts to the ethereal plane.

Round 5 (superult): The rakshasa casts 9th level fireball centered on himself.