During Chapter 4, choose one of the following towards completing the Spellhold dungeon:
Unknown Risks - Use the portal on the upper levels of the Asylum, after finding the key to unlock the door leading to it. This completes Chapter 4.
Took a Chance - Escape from the Asylum, and return to Brynnlaw. Then seek out Saemon Havarian and complete his help to secure another ship. This leads you to dealing with a new area before you can complete Chapter 4.
Both achievements can be acquired in a single playthrough by saving before completing one path, reloading, and then playing out the other path.