It is Maiev's job as warden to try and put a stop to it. There are 2 Main Objectives (The Search for Illidan and The Tomb of Sargeras) and 2 Optional Objectives ( Barriers and The Shadow Orb).

After the events of the Night Elf Campaign, Illidan escapes and tries to form an Army with the Naga, lizard like creatures from the sea. Follow him and try and stop his plans as the Warden Maiev.