This one is going to get a little weird. The Ultimate Nullifier is a part of Galactus, so much like him, it looks different to different people. The general consensus is that it looks like a little motorola cell phone. Whatever it is or does, Galactus is afraid of it. I can only think of one reason you'd be afraid of something that was part of yourself - if you had a big, you know what. Anyway, enough about that.
Generally Galactus keeps this thing with him, but he's let people borrow it in the past. He gave it to a guy named Quasar to defeat a guy named Magus, but that backfired and it worked against Quasar. It tends to float around the Marvel universe, sometimes back with Galactus and other times not, which they explain by our old friend Abraxas. Remember that whole situation where the reality was reset and Abraxas disappeared? That might have something to do with things bouncing around. You can't mess with reality.