The Final Hour

The party joins the demons of Hell in a siege against Salethar and his devils. They slip into his dungeon undetected then make their way through his castle before confronting him on his roof. The arena is covered in fey magic and scorches caused by the siege, but the party marches forward without fear. The fight ends after multiple stages and culminates back in Feyreach of all places. Salethar is defeated just before he can kill Mex.

Salethar is actually joined by Zariel as he prepares a portal back to the material plane. His ultimate goal is to return to Feyreach and merge the three primary planes- The Fey, Material, and Hell itself. Zariel's death is what gives him the power he needs to tear open the portal to Feyreach, but the party gets to him before his goals can be realized. Also, while exploring the castle the party learns of Salethar's true lineage. His father is Asmodeus himself- the king of Hell that the party thought was very much dead. Finally, they also find all tomes of the demonomicon. Not much time is spent with these books, but they do learn of Alquam's true name- Pazuzu.