Unicron

Unicron is the eternal arch-enemy of Uru the Descendant of Primus. Also known as the Lord of Chaos, the Chaos Bringer, and the Planet Eater, he is dedicated to consuming all life on Cybertron and Planet Dens.

In Transformers
According to legend, at the beginning of time, Unicron and Primus battled each other for eons, with neither side remaining victorious for long. Until, that is, Primus created the Thirteen. These Thirteen defeated Unicron and cast him into space.

Background
Unicron roamed around the continent destroying different cities and towns. In the year X774, he entered into the Land of Isvan. After destroying a few cities in the area, he traveled to Brago in the Northern Continent where he began destroying cities there; Gray, who heard that the Demon was in Brago, chased after it and attempted to fight the Giant. His mentor, Uru, came to his rescue, but realized that the Demon was too powerful for her to defeat and so she froze it through the use of a spell called Ice Make: Primus which turned her body into an ice prison that sealed away.