The Monolith Dungeon requires the player to have the Sorcerer DLC which can be purchased In the LOA Shop. This is a much exciting class with multiple summons to fight along side you.
Unless you have a group to venture into this Dungeon, you might want to be a master at your class, as these mobs tend to Heal often.
Location
Southern Hills Region. Go North of Valus Cemetary.
Xorian Items
The Monolith is the only Dungeon which contains Xorian items, which are used to build Sorcery and Summoning.