Lamia

A lamia ally is a stretch you have to sell. The plausible framings: a Charmed PC believes the lamia is on their side, the party has cut a desperate deal in a desert war and the lamia agreed because the alternative was worse for her, or a higher fiend has loaned her to the party as a one-scene asset. In every version, the lamia is lying about something, and the GM should know what. Write it down before the session.

In combat on the party's side, the lamia is a debuff specialist. Open with Corrupting Touch at DC 13 Wis on the enemy commander; a failed save means he is Charmed and Poisoned for an hour, which usually translates to "stops giving orders." Multiattack with two Claw at +5 finishes weaker enemies, and Geas once per day can end a fight by binding the surviving lieutenant to leave the region. Magic Resistance is not in this stat block, so she can be Counterspelled and Banished like any other CR 4 caster; do not let the party assume she is harder to remove than she is.

Out of combat, the lamia is the party's social weapon. Disguise Self at will lets her walk into any court as a noblewoman. Deception +7 carries any lie the party needs told. Scrying once per day lets her find a missing NPC by name. The cost: every favor she does, she remembers, and she keeps a private ledger that none of the PCs ever sees. If the party tries to dismiss her at the end, she leaves smiling and they should feel it.

Decide what the lamia wants from the party before she meets them. The ally tab works only if her endgame is sharper than theirs.

A lamia runs an oasis, a ruined temple, or a desert villa as its court, and the party walks in thinking they are guests. The fight is the third act, not the first. Disguise Self at will lets the lamia appear as a Medium or Large biped (a beautiful priestess, a kindly innkeeper, a wounded merchant), and Deception +7 plus Insight +4 means it sees lies coming and tells smoother ones back. The first scene is dinner. Combat starts when the party either sees through the act or refuses the proposal.

Open with Corrupting Touch on whichever PC stepped in closest. DC 13 Wis save, 13 average psychic on a fail, and the cursed target is Charmed and Poisoned for one hour. The Charmed condition makes the lamia's allies (cultists, summoned beasts, the lamia itself) effectively friendly to that PC for the duration, which is the whole game plan: get one party member fighting against the others, or at least standing and watching, while the lamia carves up the rest. Then Multiattack with two Claw at +5 (7 slashing plus 7 psychic each) and replace one Claw with another Corrupting Touch on the next-closest target if they're in reach. Leap (bonus action, 30 ft. for 10 ft. of movement) keeps the lamia bouncing to whoever just made their save.

Spellcasting is the campaign hook. Geas once per day at DC 13 Wis is the lamia's signature: cast it on a captured PC mid-fight to bind them to a year-long task ("bring me the heart of the king"), and the curse follows them home. Major Image once per day fakes a wall, a chasm, a duplicate of the party's patron telling them to lower their weapons. Scrying once per day means the lamia has been watching the party for days before they arrived; have an NPC mention they overheard "the lady" describing the rogue's tattoo last week.

Lamias do not die fighting if they can avoid it. At Bloodied (49 HP or less), the lamia Disguise Selfs into one of the dead cultists and crawls under a table. Let her escape. Geas on a PC is a story that pays off three sessions later.