Complex sentence: Many years later, when the undead Scourge began its invasion of Lordaeron, Prince Arthas Menethil of Lordaeron, as a death knight, began an assault on Quel'Thalas, his goal being the mystic Sunwell, the source of the High Elves' magical power.
Split into simpler sentences: Many years later, when the undead Scourge began its invasion of Lordaeron, Prince Arthas Menethil of Lordaeron began an assault on Quel'Thalas. His goal being the mystic Sunwell, the source of the High Elves' magical power.