Guardian Naga

A guardian naga as ally is the party negotiating the right to use a temple, vault, or library that she protects. She does not leave her post, ever, so "ally" here means she lets the party in, advises them while they work, and defends the threshold while they handle whatever is inside. The relationship is location-bound and you should set the geography of her shrine before the session.

In conversation she is patient and well-read. Skills include Arcana, History, and Religion at +11, so she is the best free sage in any temple-based campaign. Have her at-will Thaumaturgy be the small theatrical signal of her presence: a door bolting itself, a candle brightening when she speaks. She uses Clairvoyance once a day to scout a chamber the party is about to enter, then talks them through what she saw before they go in.

In combat she defends her threshold. If something rushes the shrine while the party is inside, she fights with Bite and Poisonous Spittle exactly as she would against intruders, only now the intruders are someone else. Cure Wounds (level 6) is one casting per day and she will spend it on whichever PC took the worst hit, no negotiation. She will not pursue enemies past the threshold. Her job is the door.

If a player asks her to leave the shrine for a single mission, the answer is no. If they ask her to lend Geas to bind a captured villain to a promise, that is exactly the kind of favor she was made for, and she will do it once.

A guardian naga is a Lawful Good temple sentinel the party has somehow ended up on the wrong side of. She is not evil and she does not hunt. She has a post (a sealed shrine, a tomb, a relic vault) and she fights anyone who crosses the threshold without the right word or rite. Open the encounter with a warning. She speaks Common and Celestial, states her charge in one sentence, and gives the party one chance to leave. If they don't, the fight starts on her terms.

Lead with Poisonous Spittle. DC 16 Constitution save, 31 poison damage on a fail, target Blinded until her next turn. Pick the wizard. Blinded gives Advantage to attacks against the target and Disadvantage on the wizard's attack rolls, which deletes most concentration spells the party would use to control her.

Bite is the secondary lever. +8 to hit, reach 10 ft., 17 piercing plus 22 poison, so a single bite averages 39 damage and can drop a low-Constitution PC outright. With Multiattack she takes two Bites and can replace either with a Spittle when the party clusters into healing range.

Spellcasting is your scene control. Geas (1/Day) is the dramatic option: if the party flees, she casts Geas on whoever stole the relic, and now they have a problem they cannot run from. Flame Strike (1/Day, level 6) is for when she is Bloodied and the party has bunched into the corridor. Cure Wounds (1/Day, level 6) is hers, not theirs; save it for after she drops below half.

Celestial Restoration is the closer. If the party kills her, she returns to life in 1d6 days with full HP unless someone casts Dispel Evil and Good on her remains. Make sure the party hears about that spell before the second time they meet her, because otherwise the naga is not a one-shot encounter, she is a recurring fact.