Jim Rook was the lead singer of The Electrics, a rock band, who entered a run-down shop named Oblivion Inc. Upon entering he and his girlfriend Janet Jones were transported to the strange dimension Myrra. Here he discovered that he was the ancestor of the land's greatest hero Nightmaster and he inherited his ancestor's mystic sword, the Sword of Night. The Sword of Night has the ability to warn of danger and force a person to speak the truth.
After his adventures in Myrra Jim opened a bookstore in the Oblivion Inc. and became active with other superheroes Primal Force and Swamp Thing before giving up on being a superhero some years later.
Jim was the proprietor of a mystical pub called the Oblivion Bar which exists inside its own pocket dimension with its own rules and laws. For example, the bar has a phone that can call other dimensions so long as you know that dimension's area code.
Jim became the reluctant leader of the Shadowpact after the team was reformed by Detective Chimp, Enchantress and Ragman to defeat a rampaging Spectre. The team helped Captain Marvel to defeat the Spectre and became the premier magical super hero team of the DC universe.
A Year in RiverRock
When a near impenetrable mystical shield appeared around the town of Riverrock the Phantom Stranger sent Jim and the rest of Shadowpact through the shield and into the town where they found a group of super powered villains calling themselves the Pentacle lead by the sorceress Strega. The Pentacle consisted of a number of villains that seemed to be perfectly suited for fighting each member of the team. Nightmaster's foe among the team was the swordsman White Rabbit who considered his killings an elegant art form and found Nightmaster to be nearly a match for him. When the team was captured White Rabbit released them after requesting that Jim use his freedom to hone his skills with a sword so that the next time they fought it would offer him true test of his skills. This release allowed Shadowpact to stop the Pentacle from raising the Sun King. The cost of doing so was the loss of a full year of their lives and the lives of all the residents of the town, though they only perceived about a day passing.
The Oblivion Bar
When they returned, Jim discovered his bar had been taken over by Edward "Flippy" Deacon, a man who possessed powerful telekinetic abilities. Eddie had reopened the bar after it had been seemingly abandoned in the year that Jim and his fellows had lost in Riverrock. Whilst Jim wanted the bar returned to him there was no legal way to repossess an establishment in its own dimension and since Eddie had nowhere else to go he did not want to give the bar back to him. Many other people could have easily been tossed out the door but Eddie could squeeze shut a trachea with a thought so the situation was at a stalemate. The pair came to a compromise that allowed Jim to stay at the bar and use one of its many rooms as a headquarters for the Shadowpact whilst Eddie ran the bar. This seemed an ideal situation for both parties and allowed Jim and Detective Chimp more time to lead and develop the team.
Jim leads the team for some time after this until one of their adventures lead them to fight the creatures known as the Unbound. This adventure leads Nightmaster, along with Nightshade and Ragman to the derelict city of Arcady where Jim had first gained the Sword of Night and his powers as Nightmaster. The once glorious capital city of Myrra now lay in ruin after the invasion of the unbound. The Unbound, it turned out, were originally the inhabitants of the city who had been infected by a magical virus of the mind which was transfered merely by hearing what were known as "the teachings of the Unbound" and caused the infected to feel no pain, fear or morality leaving them like wild animals living only for themselves and to infect others. Nightmaster was determined to save the people of the city he felt he had deserted when he had left all those years before and whilst the situation seemed hopeless he lead the team to find a way to undo the virus of the Unbound and revert the citizens back to their former selves. Nightmaster was infected by a counter spell or anti-virus created by Enchantress which instilled the virtues of courage, honesty and compassion from him to the Unbound. The spell would only work when delivered through someone with these virtues which Enchantress was certain Jim had. The anti-virus was activated when he heard the words of the Unbound and it infected all of them, through a psychic link and shared subconscious, with his virtues reverting them all back to their former selves.
With the evil vanquished, Shadowpact headed back to Earth to continue protecting it from magical threats. However, Jim decided he needed to stay as the champion of Myrra that the people deserved and to help them rebuild their city. After an emotional moment with Enchantress the team left the Myrra to Earth whilst Jim began his duties protecting his new home in Arcady fighting dragons and dark wizards.
Nightmaster recently returned to Earth and went through a portal to save his fellow compatriots. They were being held by his arch nemesis Lord Meh. Nightmaster threw his sword through Lord Meh's chest either killing him or stopping his magical forcefield enough for him to flee.
Nightmaster's powers all come from the Sword of Night which he inherited when he was transported to the city of Arcady in Myrra, in the Shadow Dimension.
The sword itself has the ability to warn Nightmaster of danger and force people to tell him the truth. It also endows the user with peak physical fitness that seems to keep the bearer's appearance at that of a young man.
The sword contains the ghost of the previous Nightmaster - Jim's father who can on occasion speak, or even appear, to Jim and offer him advice. Most recently Jim's father informed him that the Sword of Night could be whatever he believed it to be. The full extent of this power has not been fully revealed (or learned by Jim himself) but the sword has been shown to multiply into several swords, fly through the air as a projectile and cut through anything Jim believes it can.