Complex sentence: The Other Guy tells him that Laurel was lying to him about his soul: if he recovers it he will become one of the Walkers; if he doesn't, he'll cease to belong to either world.
Split into simpler sentences: The Other Guy tells him that Laurel was lying to him about his soul. Only if he takes it will he become one of the Walkers; if he doesn't, he'll cease to belong to either world.