I am not sure if this is the way to link to a closed topic maybe someone could explain to me how, but I had the very same as this thread that I linked to yesterday on an official server.
My thrall killed them yesterday and today they have not respawned, I initially thought they were someone else thralls until they started attacking me with fire orbs.
They would look like Stygian soldiers in a purge.
You probably have some neighbouring base that has initiated a purge.
The purge base probably appeared in the vicinity of your base, sometimes purges spawn randomly and create annoying bugs
e.g. you enter a dungeon and come out slapping because the purge army is in front of you:
WOW! That is alot of raiders, there is a base of another clans in the grid location so is possible that they at some point have done a purge.
My base is nowhere near this location I was going over to join my friend to the clan when I encountered these.
I just found it strange that the original poster of the link in the title had the very same issue at the very same spot but I believe was on a private server.