Frostmourne Cavern

The quest Frostmourne Cavern (8 of 8) seems to be possibly...bugged? When I enter, the place is filled with about a hundred lvl 80 undead, and I cannot get through them to the altar. In every walkthrough I've done, these undead aren't there, just a bunch of elementals.

Is this quest bugged, or is there another entrance? I've looked everywhere.

Its not a bug, those undead are in the cave because someone was on the Shadow's Edge questline but didn't finish the part in the cave. You can deal with this yourself by finding the hammer "Light's Vengeance" on the ground in the far right side of the cave and just click on it, it will start destroying all the undead.