We last saw him in a video, climbing up set of stairs before walking into a "party" filled with fog monsters. Then he screamed a bunch and vanished. Now we're getting our lore from the "House of Arkham" which is basically a group of investigators living in the observers tower. Its possible that the observer is stuck in some kind of loop by the entity of dying over and over. if you wanna know specifics just read the wiki or get the lore ingame.