RTF is a global modification that replaces all towns in the game.

I will suggest a Simurgh connected to the Void again; after all, the Academy has units related to the Void.

They even had a Void cult that created the Staff of the Underworld.
Or, you could create another bird based on primal magic, but with a different color. Judging by the images, there are three main colors: green with rays or smoke (dispels magic), purple with lightning and fog (affects the mind), and light blue with rays (deals direct damage).