Uninvited is the Ambrus cellar near St. Tyna's Grove, an outpost with ruins below it. It is the toughest of the three cellars because a named murohn guards the blood stock.
Clear the camp on the surface, go down into the ruins, kill the murohn and destroy the blood barrels. It counts toward Ambrus's anger like the other cellars.
This is the only cellar with a named boss, so arrive with healing ready. If it would not complete before, Update 1.1 fixed that.