The alliance that brought the fir'bolg into the Light armies of TFL was forged between ou'Kahn, king of the fir'bolg, and Caliban. Who's Caliban? Why is he powerful/important enough to parlay with the King of the fir'bolg and form a truce with them? And why does the truce with Caliban apply to all the human and dwarf lands? Because Caliban may be one of the avatara.
Clearly the avatara are the true leaders of the Light during the Great War - I believe this makes one of them a prime candidate for negotiating such an important truce. Also, since the fir'bolg had been "enemies of civilized nations", it reasons that there may have been some emnity between ou'Kahn and another king, whereas the nondenominational avatara don't have the interests of any single nation in mind.
On a side, but less relevant note, Caliban is the name of a mad slave from Shakespeare's "The Tempest" - the child of a witch. Also in "The Tempest" is Sycorax, one of the random Shade names.
