WARNING: The following contains spoilers for Fantastic Four #27 by Dan Slott, R.B. Silva, Juanan Ramirez, Ze Carlos, Chris O'Halloran & VC's Joe Caramagna, on sale now.
With the most dangerous enemy they've ever faced on her way to kill everything, the Fantastic Four were in overdrive to come up with a plan to stop her, but Reed Richards already had a pretty good grasp on the situation. The Griever At The End Of All Things was a near unstoppable force of nature and their best defense, Franklin Richards, had just exhausted the last bit of his godlike power. So to stop the Griever, Reed may have just asked the Silver Surfer to return the Ultimate Nullifier to Earth as a backup plan in Fantastic Four #27.
The Griever's imminent arrival meant that several of the universe's Franklin had created were gone now but their refugees needed a place to go. The Fantastic Four opened up their home on Yancy Street as a place for them to stay, but even a building with increased interior dimensions can only hold so many aliens. Knowing even their house couldn't contain literal universes' worth of people, Reed used Alicia's Cosmi-Comm, a very retro-looking helmet, to contact their old friend the Silver Surfer.
Speaking with Norrin Radd, Reed asked if the Silver Surfer could give safe passage for the refugees to one of the many worlds he discovered. The Surfer agreed with ease, but then Reed made another request of him, one that was done behind the scenes. When he finally disconnected from the call, Reed told Alicia that he had just set up his "Plan B" in the event his first plan did not succeed. Now, while there is no real certainty yet about what Reed asked of the Silver Surfer, based on the situation and the fact that it was the Surfer, out of all their spacefaring friends, that Reed turned to, then it seems possible, even likely, that Reed asked for the Surfer to bring the Ultimate Nullifier back to Earth. There is no one on Earth strong enough to defeat her and should Reed's plan to imprison her fail (which it did) then at that point, the only option left is to use the weapon that made even Galactus run with his tail between his legs.
The Ultimate Nullifier debuted in Fantastic Four #50 by Stan Lee, Jack Kirby, and Sam Rosen. It is the most powerful weapon in the Marvel Universe, which is impressive considering it can fit in the palm of a hand. The Nullifier is capable of killing beings like Galactus if wielded by a strong enough mind. But more than that, it is capable of wiping out not just the universe, but the entire timeline connected to it, from beginning to end.
The last time the Nullifier was seen, it had been handed off to Uatu by Nova. But seeing as the Watcher has only just recently returned to comics, it is a safe bet that he no longer possesses the weapon. It is a solid bet then, that Galactus is currently in possession of the Nullifier. It has been long established that the weapon can be called back to him instantly and he fears what could happen if it is away from him too long.
By that logic, the Surfer, as Galactus' former herald, might know where he is and how to retrieve the Nullifier for the Fantastic Four. A weapon so destructive it can wipe out timelines with just thought might just be the perfect weapon against a being who claims to be the embodiment of the universe's death. Her existence is part of the timeline of creation, she abides by those laws just as her victims do. So the Nullifier might just be the only thing that can end the threat of the Griever once and for all.