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.
Arthas traveled to the Frozen Throne of Northrend. Arthas broke the ice. Arthas were surrounding his master. Arthas were donning the Lich King's helm. Arthas were merging with him. The Lich King was threatened by the forces of Illidan Stormrage.