When Doctor Strange went into the Dark Dimension to save Spider-Man he was confronted by Dormammu. Doctor Strange invoked Valtorr and was temporarily able to turn Dormammu into stone by using the Vapors of Valtorr. This gave Doctor Strange and Spider-Man enough time to escape the Dark Dimension.
In the comics
Valtorr's origins are unrevealed. He was likely worshiped at some point on Earth, though whether he is closer to god or demon is open to speculation.
Eight great mystics, including Valtorr, gathered and a disagreement arose as to which was the most powerful. To settle the argument, they entered into the Wager of the Octessence: Each created a tote, with each totem holding a fraction of it's creator's vast power and possessing the ability to transform the first human who touched it into an Exemplar, a living embodiment of that power.
As many other omnipotent beings, Valtorr was involved in the War of the Seven Spheres, a cyclic conflict between magic entities which would endure for 5,000 years.
Valtorr was invoked by Doctor Strange and appeared before him alongside a number of other mystical beings, each of whom wanted to enlist Strange as a weapon in the War of the Seven Spheres, appealing to the Protocols of Servitude. To escape his would be controllers, Strange pronounced the Enchantment of Emancipation, refusing to serve Valtorr or any other being in their way.
Yoshiro Hachiman followed a calling to the Temple of Valtorr in Tahiti, where he found the Verdant of Valtorr, which transformed him into the Exemplar of Decay.
- Doctor Strange (Mentioned only)