Nessus was a member of Nyssirios tribe in Saria. He was traitor who collaborated with Blacktongues. He uses the monster Hep'naje to kill his own people.
Jason revealded the truth when he killed Hep'naje and found Nessus sing on her skin. Nessus then attacked Jason using black magic to cast ghosts on him even ghost of the Hep'naje. Nessus was defeated. Jason then cut off his head a scare the satyrs with it.
Jason met him again at Tartarus and it's player's choice to let him be or kill him and erase him for eternity.