i tried this again after reading your post and again it worked. you need 1 area to keep all the gloom creatures in, overlapping each other. add me to your neighbors and you can see what i mean. the new rat will be under one of the others. thats the one you have to banish. hope this helps.

I was really eager to try this but it didn't work for me. A real shame as I need 15 tails and seem to get one from approx every 10th rat killed. It will take forever, and a lot of energy, to complete this quest :(