Succubus

A succubus on the party's side is one of the great long cons in D&D. She is not actually on the party's side. She has either been bound by a contract she can't break, lent by a more powerful fiend who finds the party temporarily useful, or is using them as cover for a much larger scheme. Play the contract or the scheme openly to yourself as GM, and let the players fool themselves.

In play, run her as the social specialist the party can't otherwise afford. Deception +9, Persuasion +9, and telepathy 60 ft. mean she handles diplomacy, negotiation, and infiltration at a level a low-Charisma adventuring party will never match. Shape-Shift as a bonus action between any Medium or Small Humanoid lets her walk into any human-passing space. Her combat utility is real but secondary: Charm (Dominate Person at DC 15) flips a single key enemy, Fiendish Touch handles cleanup, and Draining Kiss is a finisher she will use on anyone the party leaves alone with her.

The cost is permanent. Every favor compounds. Every NPC she charms on the party's behalf becomes a witness to her existence and a liability later. Resistances to Cold, Fire, Poison, and Psychic make her hard to kill in the betrayal scene that's coming, and her 60 ft. fly speed in true form means she can leave any rooftop conversation the moment it stops serving her.

Have her develop a real preference for one party member. Not a romance arc, a hunger arc. The player will know what's happening and the table will watch the slow car crash. The night she finally uses Draining Kiss on him, in his sleep, on a perfectly normal long rest, is the night the campaign turns.

A succubus does not fight first. She infiltrates and cultivates, charming a useful PC into a slow betrayal that pays off three sessions from now. CR 4, 71 HP, AC 15, 30 ft. walk and 60 ft. fly, Deception +9, Persuasion +9, Insight +5, telepathy 60 ft., resistant to Cold, Fire, Poison, and Psychic. The numbers tell you the encounter: this is a social monster with combat as the failure case. If you've rolled initiative, something has already gone wrong for her.

When the fight does start, the opener is Charm (Dominate Person, level 8 version, DC 15 Wisdom). One PC fails the save and the party is fighting a teammate. With Multiattack she also lands a Fiendish Touch at +7 for 16 psychic on her way past. Draining Kiss requires the target to already be Charmed, so the priority is locking down a target with Charm and keeping them in 5 ft. DC 15 Con on the kiss for 13 psychic and a permanent reduction to Hit Point maximum until the next Long Rest. Two stacked kisses cripple a PC going into the next encounter.

Positioning runs through the fly speed. She drops Charm from above on the lowest-Wisdom PC, hovers at 60 ft., and lets the dominated PC tank for her. Resistance to Cold, Fire, Poison, and Psychic shrugs off most party blasters; the fight ends on a clean Radiant or Force hit, or when she Disengages out the window. Shape-Shift (bonus action, any Medium or Small Humanoid) is the getaway tool, but fly speed is true-form only.

Succubi never die fighting if they can help it. Once Bloodied, Shape-Shift to a screaming servant, blend into the panic, and exit. The party should beat the encounter without quite confirming the kill. Players who think they killed her at the noble's party will spend the next session paranoid about every NPC who flatters them.