Lord Nyax was originally a scary bedtime story told to children. Corellian children were "lulled" to sleep with tales that Lord Nyax would come to steal the young boys and girls from their beds.
After the events of the Clone Wars, the Lord Nyax story quickly became associated with Darth Vader, who also appeared (seemingly at random) to steal and/or kill children sensitive in the Force during the night before they could grow to become Jedi and become a threat to the New Order.
A cult was formed by AB 1 that worshiped Nyax and caused numerous terrorist attacks in his name. Their leader took up the name Lord Nyax, but was eventually.
During the Yuuzhan Vong War, the mentally unstable Dark Jedi, Irek Ismaren took up the mantle of Lord Nyax and fought against the combined efforts of the Yuuzhan Vong and the Jedi order.