Eldest

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

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

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