Dead Apostle Ancestor No. 10

A living mass of chaotic matter analogous to a primordial soup, created from the accumulation of the lives of 666 creatures. His name, phonetically spelled "Nero", was given to him by the Church as a play on words in Gematria by which the letters that compose it add up to 666, the so-called Number of the Beast.

Originally an alchemist named Fabro Rowan from the Sea of Estray, a cluster of smaller sorcerer organizations on northern Europe. His attempt to attain immortality led him to develop a method by which to absorb and merge his existence with the lives of other creatures, leading to his transformation into the form of Chaos. He eventually became a sole accumulation of lives with a collective consciousness, aligned with his will yet not entirely under his control and of nature beyond his own understanding. Even as he was conscious that his mind was fading by being united to the beasts, his ultimate desire was to come to understand the chaos that formed him.

(Source: TYPE-MOON Wikia)