Worldkiller

"Surtur's legendary harvester of worlds: the Worldkiller. Cross into Muspelheim and seek it out." "How will I find it?" "'Tis the harvester of worlds, Thor. Unlikely to be subtle."

- Surtur

The Worldkiller was a doomsday weapon created by Surtur, the ruler of Muspelheim.

History
Thor repelled an attack on Earth by the Fire Demons. Loki urged him that the only way to save the planet from becoming a smoking ruin was to enter Muspelheim and destroy the Worldkiller, Surtur's legendary harvester of worlds. Fighting his way through the Infernir, Thor destroyed each of the Worldkiller's supports, which led to the destruction of the Worldkiller itself.