The Ultimate Nullifier appears as a small hand-held silvery-metallic device with little or no apparent functionality, though it is possible that like Galactus, it appears differently depending on who is observing it.
The Ultimate Nullifier is capable of destroying any target the bearer directs the Nullifier at. If the bearer lacks the proper control, concentration, or power then the Nullifier will destroy the bearer as well and possibly the universe itself. While the origins of the Nullifier are largely unknown, it had been revealed that the Ultimate Nullifier is an aspect of Galactus himself. It is the only known object known to inspire fear in Galactus.
Use of the Nullifier
When Galactus first appeared to devour Earth, Uatu broke his oath of non-interference and helped the Fantastic Four. He sent the Human Torch to find the Nullifier in a secret location. Reed Richards threatened to use it against Galactus frightening him so he left earth to never try to consume it again unless he was really hungry.
It later appeared inside Galactus starship during the Infinity War. Galactus had left Thanos and a number of Earth’s heroes aboard his starship as he pleaded his case, on behalf of Adam Warlock, to allow the Infinity Gems to function as one. As soon as he left, the Magus appeared and kidnapped Warlock. Thanos was running out of options and inquired Susan Storm about the Ultimate Nullifier’s location. Nervously, she betrayed it’s location with a look. Thanos picked it up and mentioned how powerful and dangerous the weapon was, adding that it would require a far nobler soul than his own to fire it. He then hand-picked Quasar out of the group of assembled heroes and asked: "How steely is your resolve, Quasar?". Quasar took the Nullifier and in an effort to learn as much as he could about it consulted Epoch.
When Quasar arrived in the Magus’ dimension he was (unbeknownst to him) blanketed with an energy dampening field which disrupted the Nullifier, but not the Quantum-bands. When the controls of the dampening field were later damaged by Dr. Doom and Kang the Conqueror, Quasar was able to activate and slowly build power in the Nullifier, only to have the devastating effect turned against him by the Magus wielding an incomplete Infinity Gauntlet empowered by the Living Tribunal.
The Nullifier was next used when Morg took it from the hold in Galactus ship in an attempt to defeat Tyrant. The power of the Nullifier was temporarily contained by the Toady-Drone, which attached itself to Morg and fused them all together. Eventually the power was too great and it went off, destroying Galactus' ship and all on board.
Later, Abraxas, the embodiment of destruction attempted to destroy all reality with the Nullifier. But first, he needed to locate it, as it was back in Uatu's secret hidding place. But Uatu was in a coma. Since Johnny Storm was the only one to ever know it's location, he was sent to find it before Abraxas could. The other members of the Fantastic Four went into the bodies other realities versions of themselves in order to scan that reality's Torch to find all the pieces of the puzzle to the Nullifier's location. After scanning the alternate Torch's minds, Johnny was able to find the Nullifier, only to be betrayed by an alternate reality Frankie Raye, who was the Herald of Abraxas. She then gave the Nullifier to her master. When Valeria and Franklin Richards helped to resurrect Galactus, he demonstrated an ability to effortlessly call the Nullifier to his person. He then gave it to Reed Richards who used it to destroy Abraxas and all or reality, which then reset itself to normal afterward.
Alternate Futures and Realities
The Black Celestial Saga
When the Earth-616 Fantastic Four traveled to the future to stop Galactus (who had been tampered with by the Black Celestial) from absorbing the entire universe, they realized that the Ultimate Nullifier was the only thing that could stop him. Richards even theorized that the Nullifier was Galactus' emergency failsafe, created for just such an occasion when his hunger would grow to outsize proportions. They gave him the Nullifier and he used it to destroy himself.
Captain America and the Korvac Saga
In this all ages story in an unknown alternate universe, Captain America and the Guardians of the Galaxy teamed up against Korvac. Captain America was about to use it against him but didn't, because his moral code wouldn't let him kill. Korvac then grabbed it and ended up neutralizing himself.
In other media
In the video game Marvel: Ultimate Alliance, the Inhuman Medusa travels to Earth to find the Ultimate Nullifier and hopefully use it against Doctor Doom. This plan fails, as Doom expected someone would try and use it against him, and so uses his powers to brainwash Medusa.
In a side mission later on, players have to find the Ultimate Nullifier (which has been damaged). If it is found, Mr. Fantastic will one day fix it, and use it to repel Mephisto. If it isn't, then the team of heroes will have to join together again to stop Mephisto, with the resulting battle causing a massive amount of damage.