Chapter 12 Walkthrough for Baldur's Gate: Siege of Dragonspear
When you first enter into Avernus there will be a battle nearby between a group of crusaders and a group of demons. You're free to help or attack the crusaders along with the demons, the choice is yours. All of the Avernus maps are going to be really small so I won't explain where to go, as mentioned on the previous page you're pretty much on rails at this point.
On the next map you'll find a demon by the name of Thrix the Profane who teleports in via portal after Caelar goes through the large doors. There will be some dialogue with him after which he'll summon a group of enemies to fight you. Defeat the enemies then Thrix will propose an agreement with you; answer his riddle correctly and you get to pick from one of three items. Fail to answer the riddle and he will take the soul of one of your companions - or you, depending on whose soul you want to wager.
The answer to Thrix's riddle is #3, "You speak of murder, devil". The three items you get to choose from him include a Tongue of Acid (Sword), The Guardian Devil (Helmet) or Echo of the Fiend (Staff). It's up to you which of the three you choose, Thrix is most attached to the Echo of the Fiend (Staff) item and that provides the best dialogue.
Go through the door Thrix opens and you'll find a large elevator. I'd recommend resting in this area as it's the last spot you can safely rest without dealing with enemy spawns. Creating a save at this point isn't such a bad idea either since all that's left is an elevator ride up where we fight a bunch of enemies then a boss fight.
Before the boss fight starts you'll have some dialogue to go through which has a variety of outcomes. You can have Caelar join you, you can betray her or you can witness her turn into a Blackguard.
As you may imagine, this being the final boss fight of the Siege of Dragonspear Expansion Pack it can be quite difficult. Belhifet has a lot of Fire Resistance and in order to hit him you'll need to have a weapon with at least +3 on it. Admittedly, I am not very good at this game (or any CRPG) so I won't offer you much advice on the fight itself...
Once the fight is over speak with Caelar Argent or Aun Argent if Caelar is dead and you will have some dialogue plus another scene. Depending on who is left alive either Caelar or Aun will stay behind in Avernus and close the portal behind our party when we leave. This will conclude the Avernus portion of this chapter.
Exit the portal room when you get back and you'll be thrown into dialogue with Torsin de Lancie and the other Flaming Fist nearby. Speak to whoever you want in the basement area, everyone here congratulates you on a job well done and doesn't have much else to say. When you are ready to continue with the story speak with Sergeant Dazzo on the stairs and you'll be taken to your bedroom.
While you sleep you'll get a scene with the Hooded Man during which you'll have to fight one of the translucent demon monsters again. This ends up being Skie Silvershield who you murder during the scene. There will be a lot of dialogue from this point out.
During the scene where you stand trial for your crime you'll be allowed to present evidence of your innocence. This can change the outcome of what happens later on in the Epilogue if you present at least 4 pieces of evidence. If you present no evidence at all then Imoen will send a thief that breaks you out of jail; if you present 4 pieces of evidence or more then Belt himself will break you out of jail.
A few people will visit you while you're in jail as well - none of the dialogue you choose will change the outcome of anything so pick whatever you wish. When you get out of your jail cell make sure to grab your belongings on the table in the central room because we still have to do some fighting.
Now, depending on who broke you out of jail you'll start at a different point on the sewer map. If Belt broke you out then you'll have to travel a little bit to the west to find the exit and then you're free to go. Otherwise, you'll start in the sewers and you'll have to jump down the waterfall in the south to escape. If you start in the sewer portion you'll be able to grab a key from a corpse in the north too which will let you open a chest nearby the waterfall after you jump. The chest only has potions and scrolls though so you're not missing anything major.
When you reach the southern end of the sewer area you'll have a scene with Commander Corwin and Bence Duncan. They'll encourage you to surrender before eventually attacking you. It's up to you if you want to fight them or not, when you're ready to escape and proceed to the end of this area click on the waterfall and your character will jump down.
From this point on all you need to do is exit through the west zone line and you'll find Imoen waiting for you. Some dialogue will transpire after which you can follow her to the southeast where you have other companions waiting for you too. There will be a few more scenes, some dialogue and then we're officially done with the Siege of Dragonspear Expansion Pack for Baldur's Gate 1. Congrats!