Eldest

The Eldest is an aboleth of massive size, incalculable age and inconceivable malignancy. It rules the city of Xxiphu and the Abolethic Sovereignty. It is the most powerful and oldest aboleth in the city of Xxiphu and is believed to be the only "living" being that can use the Key of Stars to unlock the Far Manifold beyond Forever's Reach.

History
The Eldest is said to have been old when Abeir-Toril was split asunder.

Behind the scenes
The Eldest is based on H. P. Lovecraft's Cthulhu.