Darth Wyyrlok is a Chagrian and part of the One Sith, an order of Sith not following Darth Bane's ancient "rule of two". Darth Wyyrlok is Darth Krayt's right hand man. He serves the Dark Lord as his most trusted servant. His grandfather and his mother were both given the title of Darth Wyyrlok before him and it has been passed down ever since. So his real title is actually Darth Wyyrlok III.
Darth Wyyrlok was fully aware and fully focused on his master's disease caused by the Yuuzhan Vong bioforms. Darth Krayt was sick and dying. So of course, his most trusted servant, Darth Wyyrlok III was determined to aid his master in any way possible.
The Temple of Lord Andeddu
After Cade Skywalker escaped The Sith temple on Coruscant, the fight exhausted Darth Krayt and was forced to go back into stasis. In an attempt to help his master, Wyyrlok ventured into Andeddu's lost secret ancient temple, encountering misguided force users, who utilized the dark side energy found there. Wyyrlok was a master of dark side sorcery and used his mind tricks to defeat the misguided force users. Upon their defeat, he used one of the men as a guide into Andeddu's temple. The guide's name was Gerlun. Unfortunately, Gerlun reanimated the ancient Sith and the two men engaged in combat. But Wyyrlok, knowing that Andeddu was using illusions at him, played his own illusions back upon the ancient sith. After the battle, Wyyrlok concluded that none of Andeddu's ancient books could help Krayt, and returned back to his master side.
At this time, Darth Krayt was still dealing with the issue on Dac. He was committing genocide, killing of the Mon Calamarians to display his superiority.
Vector and the Fall of Darth Krayt
When the events of Vector hit the Legacy era, Darth Krayt was still very weak from his illness. Cade Skywalker and his band of friends were off to assassinate the Sith Emperor by tricking him to come to the planet Hab Abbadon. When he sensed his master needed his help, Wyyrlok rushed to his aid. He later found his injured master.
Darth Krayt, hanging on to life, asked Wyyrlok to get him to his stasis chamber. Ultimately, Darth Wyyrlok apparently kills his master Darth Krayt and becomes the new leader of the One Sith.
Before his death, Darth Krayt muttered these words, "My vision…my dream…for One Sith…will live!"To this comment, Darth Wyyrlok, replied, "It is a magnificent vision, my Lord. Sometimes, for the dream to live…the dreamer must die."
As leader of the One Sith
In order to hide his betrayal, he brought the body of his former master back to the planet Korriban and put his body in stasis and told Krayt's loyal followers that he was taking control of the Sith Empire while Darth Krayt would heal. He begin to change the direction of the Sith order. He appointed High Moff Veed as Regent of the empire, however the commands would come to Wyyrlok himself. Wyyrlok later contacted Vul Isen to activate a final protocol to exterminate all life on Dac using a bio-weapon as a message to the galaxy. After the extermination, Wyyrlok was contacted by Darth Nihl who was on Korriban revealing to him that the body of Darth Krayt was missing.
Shocked by these events, Wyyrlok quickly returned to Korriban to Darth Krayt Stasis, and realized Darth Nihl was telling the truth. In order to fool Darth Nihl, he told him that someone may have killed Darth Krayt and taken the body. He also told Nihl to keep this a secret between the two of them. Later, Nihl contacted Wyyrlok again informing him that Darth Talon may be the culprit. Wyyrlok told him to find her and capture her, then he returned to Courrcant to contemplate about the recent events.
Later, Wyyrlok, along with all the Sith in the Sith Empire felt a dark tremor in the Force, and it was revealed that Darth Krayt has come back from the dead.
Knowing that he could not hide this from the Sith Empire, he revealed that he did indeed killed Darth Krayt, and told them that this Darth Krayt was an impostor, because Krayt was sick and dying. He also told them that he would be the one to kill the impostor. As he prepared for Krayt's arrival, he told his daughter Saarai to flee Courrcant. Darth Stryfe appeared and told Wyyrlok that the winner of this outcome, he would follow.
Darth Krayt soon arrived and the two battled. After a long battle, it would appear that Darth Wyyrlok had won the battle, and was about to deliver the final blow, but Darth Krayt stabbed him in the chest with his lightsabers. Dying, Wyyrlok cling on Darth Stryfe, hoping he would help him, but Stryfe walked away to serve Darth Krayt once again.