Make sure none of the other side events are occurring within the pools area. It was my experience searching these out last week that they had a rotation of sorts. For me it was - "Red dot passes over your ghost" > cabal sniper nightmare "Dark ether gathers" > scorn abomination nightmare "Caiatl offers her support" > battle again Calus's champions (one nightmare each) In my experience the next one would not spawn until the previous was either killed or fled.