This is Warmaster Seerus from Spiral Knights, a new boss from their recent Expansion Mission. Probably one of my favorite missions/things in Spiral Knights so far. They did a spectacular job with the entire mission, from the story it tells to the level design to the action. If you play Spiral Knights and haven't picked it up, I highly recommend it, especially if you really dig the game's story/lore.
Warmaster Seerus © Three Rings