Okey so I've been farming this dude for nearly a week now, it just won't spawn. Me and my friend have tried everything, and I mean EVERYTHING. If there is a combination of different colors - we have tried it. Other sources state that this monster cannot be on an absurd spawn-timer either, and it must be a trigger. However I found one very easy way to do this, and it proves that one specific theory may be correct.
I logged in a few minutes ago on my zelot to continue the hunt. The first thing I noticed was this:
There where ALOT more horrors than the same time yesterday before maintainance - nearly twice as many; and often in groups of three or four monsters. I decided (knowing that whatever I did would probably not work) to thin the numbers down. After three pulls of three horrors (as many as I can take at once without breaking a sweat) there had been three horrors spawned. One of these; the third - were a Purple Horror. It spawned at the exact same place I had previously killed one of the horrors.
My theory is thus this, and it explains a lot of the mysteries people are experiencing with this monster spawn.
The spawn triggers from killing Horrors, possibly differently colored ones. However... this was after servers came up after maintainance - suggesting that this monster-trigger bugs, or is easily broken somehow.
If you're going to hunt for this one, make sure you're out early after maintainance or server crashes. Good luck



