Only because the lore states she defeated the last kell. It doesn't say she killed him though an considering she keeps a bunch in her prison of elders it's possible she has him housed in it. The raid could easily be us dropping into the prison to stop a raid force of fallen jail breaking him.

Yep, only to see he's still in his cell. But instead of reinforcing the prison or throwing the cell into a lava lake or something, we release him and lose several lives trying to defeat him. Hey, at least we did it honorably.