There’s a bug where the helltide commander dies at the end of the invasion but the completion doesn’t register. First time the bug happened the event didn’t end so I left. Second time the bug happened I waited and the event completed about 60 seconds after the commander had already dead. Just sat around doing nothing since there were no mobs spawning.
The event spawns southeast of the second northmost Lilith altar in Scosglen if my description wasn’t clear enough.