Rakshasa

A rakshasa on the party's side is an alliance of mutual contempt with a creature that absolutely intends to betray you on a timeline of its choosing. Some rakshasas have grudges deeper than their grudge against mortals, so the party offering to help kill a rival rakshasa, a celestial that wronged it three centuries ago, or a god whose temple it wants burned can earn a temporary partnership.

In combat on the party's side, the rakshasa wears a face the party chose, walks among the enemy with Disguise Self up and Detect Thoughts running, and identifies the enemy's command structure before swords come out. When the work starts, Baleful Command in the right room ends most encounters by itself: DC 18 Wis save, 28 psychic on a fail, Frightened and Incapacitated against the entire enemy line. Cursed Touch follows up at +10 for 31 average per hit, and the curse blocks rest recovery for whoever the party wants softened up. The rakshasa will not coordinate or take suggestions. It does what serves its interests and the party can keep up.

The price is always specific and always personal. It wants one secret named or one favor owed, and Insight +8 means it knows when the party is lying about the price. It accepts a lie now in order to enforce a worse penalty later. When the agreed task is done, it leaves through Plane Shift or Invisibility and the party finds the room empty.

Write the rakshasa's eventual betrayal into the campaign timeline before the alliance starts. The players should feel the clock running under every favor.

A rakshasa runs the encounter from inside the party's social circle. The fight is what happens after months of being friends with the duke, and the duke turns out to be a tiger-headed fiend who has read every letter the party sent home. Greater Magic Resistance means the party automatically fails to land any spell on it and any spell attack roll automatically misses. Counterspell, Hold Monster, Disintegrate: none of it lands. The party learns this in round one and the encounter shape changes.

Open with the deception still up. Disguise Self, Detect Thoughts, Mage Hand, and Minor Illusion are all at-will, so the rakshasa knows what each PC is afraid of before initiative starts. When the mask drops, lead with Baleful Command if it's available: 30 ft. emanation, DC 18 Wisdom save, 28 psychic on a fail plus Frightened and Incapacitated until the start of its next turn. That's a free round on the entire melee line, and Recharge 5-6 brings it back fast. Then Multiattack with three Cursed Touch hits at +10, each for 12 slashing plus 19 necrotic, and the curse means cursed targets gain no benefit from short or long rests until lifted. The party that survives this fight does not heal up before the next one.

The vulnerability is the only door in. Piercing damage from weapons wielded by creatures under the effect of a Bless spell hits past Greater Magic Resistance. Bless is the cleric's job. A blessed rogue with a rapier or a blessed ranger with a longbow is the party's win condition. If your party has neither, they have to find one before the fight, which is the hook. Truesight 60 ft. defeats invisibility and Passive Perception 18 means surprise rounds are off the table. The party must engineer the conditions of the fight.

Rakshasas do not fight to the death in a body they're attached to. At around half HP, Plane Shift to a private demiplane or Invisibility plus Fly out the window, and the duke is dead in the morning paper. Fiendish Restoration means a kill in the wrong plane just sends the fiend back to the Nine Hells with a grudge. Plan the campaign around the rakshasa coming back wearing a different face.