The Ultimate Nullifier appears as a small hand-held silvery-metallic device with little or no apparent functionality although 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 to inspire fear in Galactus.
Use of the Nullifier
When Galactus first appeared on Earth with the intention of devouring it, Uatu the Watcher broke his oath of non-interference and helped the Fantastic Four against Galactus. He sent the Human Torch to find the Nullifier, which was located at a secret location. With the Nullifier in hand, Reed Richards threatened to use it against Galactus. This frightened Galactus so much that he left Earth and vowed never to attempt to consume it ever again unless he had no choice.
The Ultimate Nullifier 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 asked 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 power of his 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 build-up was too great and it ignited, 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 again in Uatu's secret hidding place. Uatu could not divulge the location of his secret hiding place because he was in a coma. So, since Johnny Storm was the only one to have ever been to Uatu's secret location, he was sent to find it before Abraxas could. The other members of the Fantastic Four went into the bodies of other realities versions of themselves so they could scan that particular reality's Torch to find all the clues as to the Nullifier's location. After scanning the minds of Torch's from other realities, Johnny Storm of this reality was able to find the Nullifier. The Torch was then betrayed by an alternate reality Frankie Raye, who was the Herald of Abraxas. She took the Nullifier from Johnny and gave it to her master. When Valeria and Franklin Richards helped to resurrect Galactus, he demonstrated his ability to effortlessly call the Nullifier to his person. He then gave it to Reed Richards who used it to destroy Abraxas in our reality, which then reset itself to normal.
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 outsized proportions. They gave Galactus 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 the Ultimate Nullifier against him but didn't, his moral code wouldn't let him kill. Korvac then grabbed the Nullifier from Capt. America and ended up neutralizing himself.
Age of Ultron
In the Age of Ultron timeline, the Ultimate Nullifier was seen in Nick Fury's bunker. Spiderman questioned if they were 'there yet', which Tony Stark quickly informed Spiderman that they were 'not even close'.
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.