Spirit Naga

A spirit naga as ally is a temporary alignment of vengeance. She wants the same enemy dead the party wants dead, usually a rival cult, a yuan-ti faction that exiled her, or the sorcerer-king who first turned her to a fiend. She is Chaotic Evil, she negotiates in Common or Abyssal, and the alliance is brittle. Set the terms aloud and write them down: she helps with this one assault, she gets first claim on the relic in the vault, she walks away when it's done.

In a fight on the party's side, point her at the back rank. Three Necrotic Ray attacks at +6 for 21 each will end most enemy casters in a single round, and Hold Person at level 3 sets up the party's rogue or barbarian for crits on three different targets. Detect Thoughts gives her perfect read on the enemy commander's plans, which she will share with the party only if it serves her. Dimension Door lets her bypass the defenses the party would have to grind through, opening doors from the inside or simply walking out with the macguffin while the party is still bashing the front gate.

The price is the relic and the silence. Anyone who breaks the agreement is on her list, and Fiendish Restoration means she does not stay dead long enough to forget. If the party tries to ambush her after the favor is done, she Dimension Doors away and the campaign now has a recurring villain who was supposed to be a one-shot ally.

Have her drop a polite, specific compliment to the PC she plans to kill first. Players notice. They should.

A spirit naga is a vengeful sorcerer in serpent shape, and she runs the encounter as a caster first and a striker second. She has spent the years between her last death and now planning exactly how this fight will go, because Fiendish Restoration means she has died before, possibly to the party's predecessors, and she remembers.

Open with Detect Thoughts before initiative if you can spin the scene that way. The naga lurks in a temple chamber or coiled in rafters, reads the front-rank PC's mind through a wall, and chooses her opening spell based on what scares them. Round one, lead with Hold Person at level 3 (DC 14 Wisdom, three targets). Round two, unload Multiattack with three attacks combining Bite at +7 (7 piercing plus 14 poison, reach 10 ft.) on whoever held, plus Necrotic Ray at +6 for 21 necrotic at 60 ft. on the back rank. A held PC is auto-crit at melee. Lightning Bolt at level 4 is the alpha-strike option when the party bunches and Hold Person isn't there.

Dimension Door is the panic button and the trap-setter both. Use one of two daily uses to pop into a flanking position behind the cleric on round two; save the second for retreat. At Bloodied (67 HP) the naga teleports to a prepared sanctuary, and Fiendish Restoration brings her back in 1d6 days unless the party uses Wish on the corpse. A spirit naga the party "killed" three sessions ago is the right villain to bring back without explanation. She has no Magic Resistance, so a player Counterspell lands on a flat roll; her three at-will utility spells (Detect Magic, Mage Hand, Minor Illusion) are for ambush prep. Minor Illusion of her own corpse is the classic spirit naga move.

Have the naga know one PC's true name and use it. The party should suspect they aren't the first adventurers she's met.