Complex sentence: Concerned about Bilbo's ring, Gandalf investigates its origins and on discovering the truth warns Frodo that this ring is the One Ring and that it retains the evil of Sauron's spirit.
Split into simpler sentences: Concerned about Bilbo's ring, Gandalf investigates and discovers it is the One Ring. He warns Frodo and says it retains the evil of Sauron's spirit.