Complex sentence: When the Lich King was threatened by the forces of Illidan Stormrage, Arthas traveled to the Frozen Throne of Northrend and broke the ice surrounding his master, donning the Lich King's helm and merging with him.
Split into simpler sentences: When the Lich King was threatened by the forces of Illidan Stormrage, Arthas traveled to the Frozen Throne of Northrend. He broke the ice surrounding his master so he could don the Lich King's helm and merge with him.