Ilith'Bah, the Sakkara Demon

Ilith'Bah can be fought in both Single Player and Multi-player, but in different ways. In Single Player Ilith'Bah is part of the Main Quest The Heart of Ancaria. But you can still fight Ilith'Bah in Multi-player by first killing the following three Dragons: D'Cay, Lady of the Orcus, Sirithcam, the Fire Dragon and Sisslith, Guardian of Frostgard. Once all three dragons are dead go to where you would normally find Ilith'Bah (in the dungeon in the wall behind Braverock Castle). Inside the dungeon Ilith'Bah is in the following room:

There you will find a Demon Horn laying on the ground just as you would in the main quest:

Destroy the Horn by attacking it an Ilith'Bah will appear.