Complex sentence: Since then, he started to have strange dreams of cats following him for an unknown reason, where he learns, thanks to the boss of the cat army, John Doe, that his body is needed to access the real world.
Split into simpler sentences: Since then, he has started to have strange dreams of cats following him for an unknown reason. The boss of the cat army, John Doe, tells him that his body is needed to access the real world.