Complex sentence: Simultaneously, the Dwarven city of Myrgard was captured by the Ghôls (with the assistance of the Fallen Lords) and the Dwarven city of Stoneheim entombed itself rather than face a similar fate.
Split into simpler sentences: Simultaneously, the Dwarven city of Myrgard was captured by the Ghôls (with the assistance of the Fallen Lords). The Dwarven city of Stoneheim entombed itself rather than face a similar fate.