Yeah, I know he was in those episodes. I remember seeing him, I remember not knowing anything about him and I remember wondering "Why the Tartarus didn't we get shown this guy's backstory? He looks awesome, and I bet an episode focussing on him would be amazing!"
Tartarus is, in Greek mythology, the area of Hades in which those who were evil during their lifetime are tortured endlessly. It more or less corresponds to Hell as imagined in the Abrahamic religions. I prefer to use the Ancient Greek word, as "Hell" might offend some people and it was mentioned as a location on My Little Pony.