Doctor Strange, during a fight with the planet devourer Galactus, used a special spell that almost drove the giant mad. In Fantastic Four #243, written and drawn by John Byrne, a famished Galactus comes to Earth in pursuit of a renegade herald and the heroes of Earth must find a way to defeat Galactus when he decides to feed there. It is Doctor Strange that delivers the final, devastating blow.
Galactus is a primal force in the Marvel Universe, a survivor from the previous cycle of creation who devours planets for sustenance. There is speculation as to why Galactus must consume worlds, most of it focusing on the fact he maintains a sort of cosmic “balance.” He has encountered the heroes of Earth on many occasions since his debut in the 1960s and has become one of the most imposing presences in the Marvel Universe. Being a force of nature makes Galactus extremely hard to defeat, and on many occasions heroes must use outside weaponry or trickery to defeat the Planet Eater. The Fantastic Four had to make use of the Ulitmate Nullifer - one of the most powerful weapons in the universe - to banish Galactus in his first appearance. Galactus also makes use of heralds, beings he gives the Power Cosmic to, who then help him seek worlds to devour.
And it is one of these heralds that kicks the story off. Galactus is in pursuit of Terrax, a former herald. Starving and famished beyond anything the heroes have seen before, Galactus is compromised. The heroes of the Earth launch an attack on him, and while their combined power overwhelms him, he still refuses to fall.
Enter Doctor Strange, who delivers the final blow. Using the Images of Ikon spell, Strange forces Galactus to confront the ghosts of all the beings he has devoured. This manages to fully overwhelm him, causing his system to overload and almost driving him mad in the process. This ordeal, combined with low energy and the beating he took from the heroes, brings Galactus down.
Galactus is one of the most powerful beings in the Marvel Universe, and even the combined physical powers of the heroes were not enough to take him out. Instead, it fell to Doctor Strange to use magic to outthink Galactus and it worked. Even the mind of the Planet Eater was not able to handle that much at once and he fell, helping to cement Doctor Strange as a true Marvel heavyweight.