These guys are the nighttime version of the Erudite Manafiend. They spawn in the Nightfallen area of Azsura.
Right now, at least, they are always spawning as Blue quality P/S. I just fought five in a row, and all five were blue P/S. I tamed the first two, screwed up and accidentally killed the third, lost to the fourth, and tamed the fifth.
I doubt this will last. So if you'd like one, now seems like an auspicious time.