Still being assembled.
Thing is, Darklords are part of the terrain. The write-ups set them up as the Big Bads, and it's natural to want to use them that way, but it's also entirely unnecessary. You can have dozens of adventures in, say, Barovia without ever confronting Strahd directly, much less permanently slaying him and unmaking his domain. But he exists, he looms in the background, and his power and personality cast a long shadow over the entire domain. Same thing with other Darklords: think of them as part of the landscape, more akin to dark gods than mortal villains.
Keep in mind also that a lot of Darklords have the ability to return from death so you can defeat one of their schemes but it doesn't mean killing them.
Alternatively, a Darklord can show up on the scene and complicate things so the plan doesn't become kill the Darklord but survive it until you complete whatever objective you have.