Melissan is a significant non-joinable NPC in Baldur's Gate II: Throne of Bhaal. She is first encountered in the city of Saradush, where she has gathered virtually all remaining Bhaalspawn in an attempt to protect them. This attempt becomes their undoing, as Yaga-Shura, a powerful fire giant Bhaalspawn, lays siege to the city to wipe out his siblings. Melissan encourages the player to help those trapped in Saradush, first by stopping the city's mad ruler, Gromnir Il-Khan, who has sealed all the gates, and then by stopping Yaga-Shura and ending the siege. She spends most of the first part of the expansion offering you aid and assistance, and tells you about The Five, a group of five very powerful Bhaalspawn who must be stopped. She guides you to places where you can get assistance, and names several - but not all - of The Five. Other NPCs will speak of Melissan, either with respect or derision, but her own agenda is not clear until towards the end.
In a scene in the player's pocket plane, Melissan is revealed to be Amelyssan the Blackhearted, former high priestess of Bhaal
, and the one set in charge of his resurrection. However, she plans to usurp the dead god's power for herself, and has collected all the power that has been released by the death of Bhaalspawn. By the end of the game, the player character and (possibly) Imoen
are the only remaining Bhaalspawn, and the player is forced to confront Melissan at the Throne of Bhaal in Hell.
Once she is defeated, a Solar reveals that she has welded the essence of Bhaal to her very soul. She is destroyed as a result.