Thanos was once trapped outside the multiverse and all of existence after dying ins Secret War and was only able to return because Galactus accidentally freed him. When the Ultimates tried to fix time and venture into dangerous territory outside of the Omniverse, they were saved by the former Devourer of Worlds, who was then known as the Lifebringer, who created a rift back into the multiverse - while accidentally freeing the Mad Titan by mistake.
The Ultimates, consisting of some of the greatest cosmic heroes in the universe including Blue Marvel, Captain Marvel, Black Panther, America Chavez, and Spectrum, were tasked with taking on the biggest threats in the galaxy. In the arc, Galactus was changed into the Lifebringer, whose powers were reversed as he was able to restore planets and provide life with an invaluable amount of energy at his disposal. After a mission by the Ultimates goes wrong, Galactus the Lifebringer arrived to save the team from a dark fate - but in the process, he allowed Thanos to return to the Marvel Universe.
In The Ultimates #5 by Al Ewing, Kenneth Rocafort, Dan Brown, and Joe Sabino, Galactus is forced to save the cosmic team after they venture into the deep nothing of space in order to assess the damage caused in the multiverse. In the epilogue of the story, it's revealed Thanos, who was killed in Secret Wars by Doctor Doom, had been stuck in the void of nothingness when the multiverse reformed and found a new love in nothing. But, when Galactus arrived to save the Ultimates, he ripped open a portal to Eternity, which the Mad Titan used to make a shocking return to the main Marvel Universe.
Galactus didn't know he accidentally freed Thanos, but he was unintentionally responsible for bringing Marvel's most dangerous villain back from the void of nothingness. Upon returning, Thanos would continue his quest for power and revenge, killing War Machine in Civil War II and later fighting against his son, Thane.
As Lifebringer, Galactus was at his most heroic, as his powers restored planets he previously eradicated of life. However, when the Ultimates went on an extremely dangerous mission and were nearly stuck in the void of darkness, Galactus was forced to save them, while creating a portal that would lead to Thanos' shocking return after dying at the hands of Doctor Doom. Without Galactus' intervention with the Ultimates, Thanos would likely still be stuck outside of life in nothing. Unfortunately, he was freed by complete accident and has continued to wreak havoc on the Marvel Universe ever since.