Simplify the following text for a Grade 3 native English-speaking person.
Original: Gandalf tricks Bilbo into hosting a party for Thorin and his band of twelve dwarves , who sing of reclaiming the Lonely Mountain and its vast treasure from the dragon Smaug .
Simplified: Thorin 's father was their king , the King under the Mountain . But one day the dragon Smaug came to the Lonely Mountain and killed all of the dwarves he found .