Because Bungie needs a back up in case they run out of ideas for raid bosses. All you need to do is add any legendary sounding name after "Taniks the..." and you got your boss. I was hoping for "Taniks, b*tch of the Disciple", or "Taniks, the Undead" as the raid boss, but alas, we got some Sheldon Cooper looking twit with a Bruce Lee complex as the final boss