Durge/Gortash, plus a bit of Haarlep in the mix
Set in a world where Gortash escaped HoH by bargaining his body to Haarlep, as per Tav.
Durge and Gortash infiltrate the Hells to steal the Crown, when they come across some problems. Haarlep happens upon them and offers his help in exchange for a night with the Bhaalspawn. Haarlep uses Gortash's body to sleep with Durge just to mess with their minds.
Durge is unaware of Gortash's bargain, thinks Haarlep is just helping to facilitate the night by shapeshifting into his secret crush, and doesn't know that Gortash can feel every second of it in the room next door. Gortash is aroused as hell but at the same time he is furious at Haarlep for trying to mess with him this way and is insanely jealous, as he has been pining for Durge for a long time but has stopped himself from initiating anything in fears of losing control of the alliance (also probably thinks that Durge is not sexually interested in him and that Haarlep forced Durge to make love to his image just to make him feel pathetic - throwback to how the HoH denizens loved to belittle and torture him).
The awkwardness afterwards, Gortash trying to hide the truth but maybe Durge finds identical bite/claw marks on his body and puts two and two together and loses control, etc etc.
Bonus points for default white Dragonborn Durge/ if Durge gets carried away and nearly kills Haarlep in bed