When you defeat the guardians, you will trigger the sixth - and final - quest of the Act. It is given by Qual-Kehk or automatically once you reach the Throne of Destruction level of the final dungeon.
There is nothing specific your character must do other than to kill Baal and end the game. Use the stairs found on the Arreat Plateau to enter Mount Arreat and the Worldstone Keep. Descend down to the third level of the Keep and through the Throne of Destruction level.
As you make your way through, Baal will laugh at you occasionally, which can be a little unnerving. You’ll find him in a room at the end of the Throne of Destruction, but as you might expect killing him isn’t going to be straightforward! Baal sits on a throne in the final room, but between you and him are several randomly generated monsters which you must kill. When you do so, Baal will then throw a glowing ball of lightning which will land somewhere on the floor and explode. From the explosion will come another bunch of monsters and a SuperUnique boss.
Once you have killed this wave, Baal will launch another glowing orb from which another wave of minions will appear and attack. This will happen five times in total; each time will have different and more powerful monsters to fight. The minions and bosses are as follows:
- -Warped Shamans led by Colenzo the Annihilator
- -Death Mages and Unravelers led by Achmel the Cursed
- -Council Members led by Bartuc the Bloody
- -Venom Lords led by Ventar the Unholy
- -Minions of Destruction led by Lister the Tormentor
When you have defeated them all, Baal will flee into a portal behind his throne. Use the opportunity to return to town and replenish your healing and mana potions before following him through. You will enter the Worldstone Chamber and into a final showdown with Baal.
Baal is a strong opponent with a vicious melee attack and the ability to cast powerful spells, including the ability to replicate himself, although the clone has fewer hitpoints. He can also teleport away from you.
Once you finally defeat him, you’ll be greeted by Tyrael who will open a portal which you click on to finish the game and see the end cinematic trailer where the archangel destroys the Worldstone. You’ll also receive a title depending on the level of the game you have played – Slayer, Champion or Patriarch/Matriarch. If playing in Hardcore, the titles bestowed are Destroyer, Conquerer and Guardian.
Congratulations! You’ve completed the Diablo II expansion pack, Lords of Destruction. Now you can sit back and wait for Diablo III or go back to the start and do it all again on a higher difficulty!