Hellbent is an ability word.
It is found only in creatures belonging to the Rakdos cult of Ravnica. This ability activates a special bonus only if you have no cards in your hand. The ability came with some controversy because it encouraged "bad play" by eliminating all of the cards in your hand. It was reminiscent of an older time when Cursed Scroll dominated the tournament scene, and madness was already powerful enough without more help. Indeed, it did become powerful, but didn't really break the game. Demonfire came to be one of the more powerful Hellbent cards because it was strictly superior to Disintegrate. Once you got Hellbent, there was little you could do to stop it from killing you.
Primarily, the mechanic lent itself to aggressive decks. Since Rakdos was a black/red guild, those decks had no trouble blowing up whatever was in its way and finishing the job with high-power creatures. Anthem of Rakdos was an excellent finisher in such decks because it acted like Glorious Anthem does, but giving far more bang from one card.
Hellbent made another appearance in Future Sight, as did other Ravnica mechanics. This time around, it wasn't the primary focus of anything, but simply offered up an extra benefit.