Jim Rook was the lead singer of The Electrics, a rock band, who entered a run-down shop named Oblivion Inc. Inside, he was somehow transported to the strange dimension Myrra, where he was the descendant of the mighty Myrra warrior Nacht.
He took his ancestor's Sword of Night, a sword with the ability to warn of danger and force a person to speak the truth (much like Wonder Woman's lasso), and became involved in the conflict between Myrra and the evil warlocks. After his adventures in Myrra, he opened a bookstore in the Oblivion Inc. and became active with other superheroes Primal Force and Swamp Thing.
Starting with Day of Vengeance, he became active again as the leader of the Shadowpact.
Before the Riverrock, Wyoming incident, Jim was the proprietor of a mystical pub called the 'Oblivion Bar'. It existed in its own dimension, with its own rules and methods. For example, the bars phone does not work unless the dimension's area code is known.
When the Shadowpact team entered the mystical shield around Riverrock and defeated the murderous Pentacle, they lost a full year of their lives, though they only perceived about a day passing. Back 'home', Jim discovered his bar had been taken over by a younger man, who possessed powerful telekinetic abilities. Many other people could have easily been tossed out the door but the new 'owner' could squeeze shut a trachea. Jim handled this situation subtly, mainly by ignoring him and failing to convey needed information. Jim became swept up in other events; mystical assassins attacking and other needs, mainly organizing his team into a more effective fighting force.
Recently, it was revealed that the longer he's in possession of the sword, the less he ages. Rook learned that his sword is inhabited by its last wielder, his own father. 
- Although this character was originally introduced during DC's Earth-One era of publication, their existence following the events of the 1985-86 limited series Crisis on Infinite Earths remains intact. However, some elements of the character's Pre-Crisis history may have been altered or removed for Post-Crisis New Earth continuity, and should be considered apocryphal.
- 58 Appearances of James Rook (New Earth)
- 15 Images featuring James Rook (New Earth)
- Quotations by or about James Rook (New Earth)
- Character Gallery: James Rook (New Earth)
- Nightmaster at dccomics.com
- Nightmaster at Wikipedia.org
- Nightmaster at the Unofficial Guide to the DC Universe
- DC Comics Encyclopedia