The Avatar of Gaea is the boss for the new Lake Foscari map. This boss appears at minute 25 and looks like a deer/elk. Defeating the boss will unlock the Hyper version of the map.
For your first run, I recommend using Queen Sigma to find the coffin in the map (press pause to see a ? On the map) to unlock the Eleanor character for an additional achievement. By minute 25, you will be well-prepared for the boss and can do separate runs with the new character you unlocked in the coffin.