Complex sentence: As depicted in the story "The Quest of Erebor", Thorin met with Gandalf in Bree shortly before the quest began, who persuaded him that stealth, rather than force, was needed to infiltrate Erebor; they would therefore need a burglar.
Split into simpler sentences: As depicted in the story "The Quest of Erebor", Thorin met with Gandalf in Bree shortly before the quest began. Gandalf persuaded him that stealth, rather than force, was needed to infiltrate Erebor; they would therefore need a burglar.