Complex sentence: The island is also rumored to be a hiding ground for werewolves which roam free at night, although such claims are impossible to prove, given that unsuspecting visitors either leave in fear or never return.
Split into simpler sentences: The island is also rumored to be a hiding ground for werewolves which roam free at night. However, such claims are impossible to prove given that visitors either leave in fear before a close encounter or never return.