Not much is known about the Great Lion, and perhaps more will be revealed in the future by the developer of the mod. Currently, he is known to be a deity worshiped by Morwaith. According to their speech banks, one day he will devour the entire Middle Earth. They also have a tradition that newcomers must face an adult lion in order to gain the respect of the village.
Mevans, the creator of the mod, has a skin that resembles a lion. As Morwaith are his own created faction, the Great Lion might be associated with him.