The essence of the "Mad Ancient."
The essence of Xel'lotath is also known as the Sigil of Xel'lotath. This artifact possesses the appearance of a "warped angel." It is one of the essences Pious picks at the beginning of the game and determines which Ancient he serves. Like all of the other essences (also called artifacts in the game) it contains a significant amount of the Ancient's energy that it is aligned with. Alexandra Roivas comes across this artifact later in the game, and uses it to summon her Ancient, if it was not already chosen by Pious at the beginning of the game.