Cold Iron's Sins
War wages in the Material plane. The Demon King, Sipexhur, has possessed the most powerful magic user in history, Micah, and seeks to dominate the plane. 2 factions fight to stop him, Numeria, the land of untold powerful technology, and the Rebels, the last remnants of resistant beings fighting against Sipexhur's grip.
The Demon King has captured a group of adventurers, and demands they undertake a simple yet daunting mission, go to an alternate timeline, find an elf named Dasphemy that had been banished there, and bring him back, so as to further increase Sipexhur's power over the realm.
As resistant as the adventurers are, the Demon King has captured their loved ones as an incentive. The party has no other choice now, find Dasphemy and potentially doom the material plane, or doom themselves.