Complex sentence: They discover the Witch has taken Mr. Tumnus, and meet Mr. and Mrs. Beaver, who tell them about Aslan, who intends to take control of Narnia from the Witch.
Split into simpler sentences: They discover the Witch has taken Mr. Tumnus, and they meet Mr. and Mrs. Beaver, who tell them about Aslan. According to the beavers, Aslan intends to take control of Narnia from the Witch.