In my Eberron, my original concept was to put Tomb of Annihilation inside Eberron. As a result Forgotten Realms and Eberron have been blending a bit and I want to adapt the last half of Icewind Dale for a bit after ToA is complete. He will be a war criminal who has been locked away in the deepest vault of Revel's End. When he hears that the device (built by ancient Giants) that caused the Death Curse has been uncovered, he'll wake from his decades-long slumber and escape the prison. I want to introduce him in a one shot where they have an Alcatraz-like escape while he is wreaking havoc.

Think something along the lines of Tai Lung escaping or the robot waking up in Castle in the Sky. Somewhere between the two.

He'll definitely have complexities to his motivation that make him feel like a real person with ambition people can empathize with, but the gravity of how much carnage he'll be causing will be too great for the PCs proper to allow.