What causes Gandalf to decide that it is time to leave the quest?

Gandalf had to leave the Quest because the White Council were about to drive the Necromancer from Dol Guldur in southern Mirkwood. The Necromancer was actually Sauron. No-one had known who it was before that and believed it was one of the Ringwraiths. Gandalf hsad to deal with the Necromancer on his own.

Gandalf had to leave the Quest because the White Council were about to drive the Necromancer from Dol Guldur in southern Mirkwood. The Necromancer was Sauron; although it was Gandalf who previously discovered that when he secretly entered Dol Guldur. No-one had known who it was before that and believed it was one of the Ringwraiths.