There are two known prototypes of Skybird Trill. Both are completely different levels to the Skybird Trill of the final game, and neither level made it into the final game, not even under another name or level number.
This version of Skybird Trill is the 20th and final level in at least one Dungeon Keeper prototype. The objective is simply to destroy the Blue Keeper; there is no Avatar or any mention of him.
This is a strange level with a large body of water in the centre with two Portals and some neutral creatures. There are two hero-controlled areas and an oddly-shaped Prison near Blue's dungeon. Blue has two gem faces, whereas the player's only source of income after depleting the Gold Seam to the East is the Gold and Gem Seams in the cavern towards Blue's dungeon. When you reach the Gems you will be ambushed by a party consisting of 3 level 5 Wizards and a level 8 Thief. At first it may look as if there's barely any room to build anything, but it is, with care, possible to build efficient 5x5 rooms.
In another Dungeon Keeper prototype, this level is level 20 and called Skybird Trill. However, the fact that it has a Transfer Creature special means it cannot have been intended to be the final level. The objective is to defeat the three enemy Keepers, and there is again no appearance or mention of the Avatar. Money is very tight; each player starts with a somewhat limited amount of Gold (and due to a scripting error, Yellow starts with no Gold at all), Gold Seams are scarce, and there are no Gem Seams. You likely will not be able to train creatures to high levels. In another Dungeon Keeper prototype, this was level 21 and the level above was level 20 Skybird Trill.
Treasure Room - East of Red's dungeon. Reaching this will trigger a party of Mountain Dwarves entering the room beyond the door, another one after 5000 turns, and a party of two level 8 Thieves at the Treasure Room who will try to steal your spell books.