Amelyssan, also known as Melissan or Amelyssan the Blackhearted appears as a well-intended woman when she first met in Saradush, a city under siege by Yaga-Shura, a powerful Bhaalspawn and one of the Five, who is seemingly invincible. She and a group of civilians are trying to gain an audience with Gromnir Il-Khan, a Bhaalspawn orc who is a veteran general and has locked himself up in the castle. The guards at the door refuse this audience and will turn hostile towards the party and the civilians.
Melissan recognizes Gorion's Ward as a child of Bhaal, as she has met many Bhaalspawn during her journeys to bring them to Saradush, for an unknown reason. She will ask you to help defend the city and defeat Yaga-Shura before the walls are breached. Your first task is that you must find a way into the castle and convince Gromnir to stop his soldiers from making the situation worse than it already is. After infiltrating the castle and going to the council chamber, the paranoid Gromnir accuses Melissan of being in league with the Five and bringing as many Bhaalspawn as possible to Saradush, not as a sanctuary, but to make the killing of rival Bhaalspawn as easy as possible for the alliance of the five most powerful Bhaalspawn. Gromnir also believes that the party of Gorion's Ward are assassins sent by Melissan to kill him, which makes it impossible to avoid bloodshed.
Now you have dealt with Gromnir, you must find a way to defeat Yaga-Shura before he has breached the city gates. After you have obtained the means to kill Yaga-Shura, the walls of Saradush are breached and you must defeat him. After he is defeated, Mellisan appears once more and tells you that she will meet you in Amkethran, where she will acquire the locations of another two Bhaalspawn, who are members of the Five. However, when you arrive there, she has moved on, but a local head of the monastery, Balthazar, will give you the locations.
When the last member of the Five is killed, Solar will summon Mellisan to the Pocket Plane. Then you will find out that she is in fact Amelyssan the Blackhearted, a chaotic evil high priestess of Bhaal and Deathstalker and that she was the most trusted follower of Bhaal. She is the only one who has access to his essence of his many children. As more children lost their lives, the more essence she gained and the more powerful she became. She has deceived the Five and with false promises of becoming powerful servants of Bhaal, used Gorion's Ward to do her dirty job of killing the Five for her, and she has deceived all children of Bhaal who were brought to Saradush, which was not a sanctuary, but in fact a deathtrap when Yaga-Shura started to siege this city. This means that Gromnir Il-Khan's paranoid ideas were true after all.
But she has betrayed Bhaal as well, as she refuses to start with the rituals required to resurrect him, only to become a goddess of the portfolio of murder herself. She only wants to kill you for your essence, as you turned out to be the most powerful Bhaalspawn of them all. After overcoming the challenge of The Ravager, you must use the teleporter. You will be transported to the actual Throne of Bhaal, which is the start of the final chapter. You will fight Amelyssan, who has started absorbing Bhaal's essence, which has made her very powerful. The battle consists of four battles with Melissan, with three fights with, in order, the Prince of Elemental Air, three Greater Air Elementals, and two Vampiric Mists, then a battle with two Frost Salamanders, the Prince of Ice Elementals, and a couple Greater Ice Elementals, then a battle with a lot of demons, interspersed within. You cannot rest there.
If you are in dire need of a rest, there is a loophole that can be used in order to regain spell slots and equipment properties while in the final battle. Using the 9th level spell "Wish" in conjunction with potions of insight, one can repeatedly use Wish until the option appears to have the party be restored as if they had rested for 8 hours. This is a fairly rare option that the Wish spell can present, so it is advisable to re-load the save file after every attempt until the affect is achieved.
Additionally, it is possible to do massive damage to Amelyssan before she begins to battle the adventurers in the beginning of each of her later phases. With the use of a rogue, one can set up to six traps in the area where Amelyssan will appear in after the party deactivates the pools of energy she is drawing from. She will be damaged immediately upon appearing in the cutscene, making the ensuing battle much easier. The use of high-level ability traps, especially spike trap, are even more effective.