Main floor of the Spellhold asylum. You'll be given a short tour around the floor upon arrival. Imoen is here, but she's not her usual self. Later you'll be thrown into the labyrinth, where you have to fight your way back here. Trace your way back to Brynnlaw to accept Saemon's offer for a bonus area, otherwise use Jon's Key to find a portal in the basement which you may enter to directly descend into the Underdark.