The Deep

The Deep is a concept in Dark Souls III.

Description
Similar to the Dark Sorceries, the power of the Deep consists of the sediment of the Dark, called dregs. It also has the capacity to generate life, as if it were stagnant waters, and is drawn towards living things.

Lore
What it is exactly is uncertain, but it seems to start in Irithyll with the cleric Aldrich, who experienced visions of the deep sea. This lead him to discover another nature of the Dark, wherein the dregs of it will sink to the lowest depth possible, and form the core components of the world. This likely lead to his departure from his home, as his visions of the Dark would be anathema, considering the nature of the city. After his departure from his home, he founded the Church of the Deep on his visions, revering and utilizing the Deep, and with some amount of time the Cathedral of the Deep was a known and peaceful religious center, overseen alongside Aldrich by his archdeacons (Royce, McDonnell, and Klimt) and fellow Irithyllian known as Sulyvahn. But the power of the Deep is far darker then what exists in humans, and the dregs built up and began to breed abhorrent life. Soon this once peaceful place became the home to things most foul, and the deacons were required to protect themselves from it. Some of those who slipped into the Deep began to wallow in this filth, while others were steadfast and resisted the maddening corruption to some meaningful degree, though they still began to actively practice dark arts. Despite all this, it appears that Aldrich was immune to the madness, and for this and his visions he was declared a saint. But the sediment of the Deep began to manifest their nature, swollen things drawn towards life, through Aldrich. He was essentially the Deep's incarnate, as he began to consume human flesh, reveling in the screams of those he would consume. His followers began to supply him with sacrifices, and even began to gather children to feed to him Eventually he consumed so many that the dregs of the Dark built up within him, before it collapsed onto itself, transforming Aldrich into a mass of dregs and his own rotting remains. Despite this sorrowful state, he remained sentient and sane, further proving his sainthood. Sheltered by the clergy, he became massive, sustained by flesh and filled with endless bones.

His power made him an ideal candidate to link the First Flame, but whether he was forced to or chose to, he was worthy only for the power of the Deep within him. When he revived and learned of the world state, he experienced more visions about the inevitable age of the deep sea, and neglected his throne. His visions showed him the last of the old royalty of Anor Londo, and resolved with courage in his heart to devour all bearers of the Light Soul, possibly to catalyze the Age of Dark. Leaving Royce and the main clergy to keep watch over the Cathedral, and their saint's coffin, he left for Irithyll accompanied by McDonnell and Sulyvahn. Using the power of the Profaned Flame, Sulyvahn overthrew the rule of the gods and declared himself Pontiff, fulfilling his ambition to become ruler of the old city. He then imprisoned Gwyndolin, who sacrificed himself to save Yorshka, but was left to be devoured by Aldrich. His powers overtook the moonlight as he slowly consumed and controlled the god, and he began to dream, from which he developed a miracle to fight with.

Trivia

 * Next to the Cathedral, there is a black void that looks like a glitch, but is watery in appearance when closely observed. It could be that this is where the dark of the Deep is located in the Cathedral.