Complex sentence: He was a fearsome dragon who invaded the Dwarf kingdom of Erebor 150 years prior to the events described in the novel; a group of 12 Dwarves mounted a quest to take the kingdom back, aided by the wizard Gandalf and the Hobbit Bilbo Baggins.
Split into simpler sentences: He was a fearsome dragon who invaded the Dwarf kingdom of Erebor 150 years prior to the events described in the novel. A group of 12 Dwarves mounted a quest to take the kingdom back, aided by the wizard Gandalf and the Hobbit Bilbo Baggins.