Several millenia ago, Odin created the monster Mangog from the soul hatred of billions of souls to defeat the rebellious Vanir. When he discovered that the creature was too dangerous to be controlled, Odin imprisoned it in the Cave of Ages, intending that it would stay locked up forever.
After his friend Sif was killed by a Frost Giant during an attack on Asgard, Thor traveled to Niflheim seeking vengeance. Loki revealed the Cave of Ages to him, informing him that it contained a weapon so powerful it could melt all of Niflheim. Thor noticed carvings on the entrance in the lost tongue of Odin, and wondered why the Allfather would hide such power in that frozen Hel. Loki suggested that Odin hoped for a peace that would never come and that he had lost his nerve. Ymir warned Thor not to open the cave or the Nine Realms would suffer for it. Ymir attacked Thor, vowing not to let him bring the Apocalypse. Thor opened the gate and the burst of power that came forth knocked him off his feet and melted Ymir. Mangog emerged and set out for Asgard to inflict vengeance upon Odin.