The Underdark | Adalon's Lair | Ust Natha | The Beholder Dungeon | The Mind Flayer Dungeon | The Kuo-Toa Dungeon | The Exit Area
The Ust Natha Tavern | Female Fighters' Society | Male Fighters' Society | The Temple of Lolth | Qilue's Home | Deirex' Tower | House Jae'llat
To enter here you must have the Magical Rope from Visaj at 14 in Ust Natha.
The first time you enter, the Lich immediately casts Imprisonment on you. However, you are whisked away to Jarlaxle's stronghold and he sends you back to immediately outside the tower. Thus, before entering for the first time you don't need to put up any magical defenses. But this is strongly recommended before entering the second time. ;-)
You can only gain access to the second floor after you have killed Deirex the Lich and taken the Lich's Tooth from him.