While something of a demon in appearance when encountered by the player, Manus was in fact once human. He became the Father of the Abyss when his humanity went wild, but it is not known what caused this. According to both Marvelous Chester and Hawkeye Gough, an unidentified Primordial Serpent (whether it was Kaathe or Frampt is uncertain) urged the citizens of Oolacile to delve in the dark of the Abyss, where they awoke this primordial man. Upon awakening, he raged and set to expand his domain while seeking out his precious Broken Pendant. His influence consumed the citizens of Oolacile, whose humanity went wild in turn, mutating them into ghastly creatures. Its inhabitants gone mad, Oolacile was left to fall to ruin. Nothing remains in present day to mark the passing of Oolacile, save Dusk.
When the player finds half of his pendant, Manus reaches out through time and space to reacquire it, taking the player to Oolacile in the process. He also takes Marvelous Chester in the same manner, who may have found the other half of the pendant in another time. While this pendant obviously held some deep meaning to Manus, but it is long since lost to time.
Sometime before the player's arrival into the past, Artorias and his young wolf Sif faced off against Manus. However, Manus overpowered them. Artorias sacrificed himself to protect his compatriot. Manus, having defeated Artorias, corrupted him against his will.
After his defeat, his soul split apart and became fragmented. Eventually some of these soul fragments became self-aware and formed into their own entities.
'Manus' is the Latin word for 'hand'.
Additionally, Manu is a term found with various meanings in different mythologies of Hinduism. In early texts, it refers to the archetypal man, or to the first man, progenitor of humanity. In later texts, Manu is the title or name of the head of mythical dynasties that begin with each cyclic aeon when the universe is born anew.