Nyx is one of the six mages who ruled Atlantis, and is Ava's ancestor.
History
Pre-series
Nyx came from the shores of the Missisippi. He was blind, but felt things instead, the same way a bat uses sonar or sharks and marine mammals using echolocation. Magic strengthened his gift so that he not only could he see what already was, but things that had yet come to pass. His descendant Ava would inherit his gift.
He was killed by Abbadon in the sinking of Atlantis, before he managed to enter the temple, but he found an injured Navi, who made it with Nyx's talisman and her own. The surviving mages managed to drive Abbadon into the Carceron, but out of the Six Who Ruled, only Merrow survived. After his death, Merrow honoured Nyx's memory by throwing his talisman, the ruby ring, in the Mississippi River where he was born, as it was his talisman. The ring was protected by the Okwa Naholo, ghosts of a cruel Choctaw warrior and his men, who are so horrific to look at that the mere sight of them would stop anyone's heart.
Deep Blue
Ava, his descendant, comes to the Iele's Cave along with the other descendants of the Six Who Ruled. Baba Vrăja, leader of the Iele, shows them a memory which was passed down from Merrow after she established the River Witches. Nyx is shown explaining to Merrow that Orfeo had barricaded himself in the temple of Morsa. She tells him that they had to get his talisman and all the others'. The plan was if they could all get them together, they would be able to force the monster Abbadon into the Carceron and seal it, using the talismans. Nyx replied that Orfeo would never surrender the talisman, and that they would have to kill him in order to obtain it. Merrow replies then, they had no choice.
Nyx is shocked and horrified, begging Merrow, reminding her that this was their friend, and trusted mage Orfeo, but Merrow realised that there was no other way and that Orfeo would kill everyone in the island. She left to get Sycorax and Pyrrha, the other mages and Nyx needed to meet them in the temple after he'd found Navi. Baba Vrăja explains that Nyx died before he entered the temple, but not before he found Navi and gave her his talisman, so she could do what needed to be done.
Dark Tide
Serafina, Astrid, Becca and Ling learn, via convoca from Ava that Portia, Vallerio and Traho learned where the ruby ring was- in the Mississippi. Sera panics and tells Ava that it was too dangerous to go after it, but Ava insists, mentioning that she had fallen ill when she was young and her sight had been taken from her. At that time she didn't understand why, but she had been told there was a reason for that. The reason was that she would retrieve the ruby ring.
Sea Spell
Ava goes to Manon Laveau, the Swamp Queen who heals her fever, feeds her and gives her a bag of gris-gris in order to survive the journey. Manon Laveau told Ava that the mere sight of the Okwa Naholo was so horrific it would kill her but Ava mentions that it was a good thing then, that she was blind. Ava retrieves her ancestor's ring with her guidefish Baby, but soon afterwards, is captured by Traho, and Baby was killed. Nyx's ring was taken by Traho who delivered it to Orfeo, who used it to open the Carceron.
Orfeo mentions to Astrid that the other mages thought that they had succeeded in driving Abbadon back, but it onl retreated into the Carceron because he told it to.
After the battle with Abbadon and Orfeo, Nyx's ring is taken along with the other talismans (minus the emerald and black pearl of Orfeo) to be sealed away.
Appearance
Nyx is described as "regal", being slender and dark-skinned with blind eyes and high cheekbones and black hair in dozens of long braids. Ava inherited his complexion, cheekbones, hair and eyes.
Talisman
His talisman is a ruby ring, which was hidden in the swamps of Missisippi, guarded by water spirits called the Okwa Naholo.