As the name says then getting this kill was literal a nightmare.
After we had used some time on Nythendra and found out how our team did on HC we choose to do Dragons of Nightmare. These were rather troublesome to kill as it took us around 3 different raid lockouts and thus got 3 different dragon combination. But these were not our only problems as one of our healers was on vacation and another left the raid team thus leaving us in a rather big of a pickle, which forced us a bit into doing normal 1 of the weeks. But as our healer returned from vacation and a dps had gone a bit healer (so we had around 3.7 healers) we could go back to clearing HC. Another problem we had was bad transition as we usual ended up getting the dragons special ability just as a new dragon appeared or dragon switch happened thus usual resulting in lots of extra dmg or stuff to kill.
But after a few nights and lots of tries we finally killed the dragons. Sadly this is not the end of the nightmare, but just the start.