In Gears of War 3 for the Xbox 360, you can unlock the Comet mutator, which will build up energy as your Roadie Run that you can use against your enemies, but cut your experience gained by 25%.
To unlock the Comet mutator, you need to earn 1,000 First Blood ribbons. This one will take a lot of grinding, and may come naturally if you play a lot of games, but is probably easiest with Private matches filled with bots set to really short rounds.
Check out my other guide if you’re looking for the full list of unlockable mutators in Gears of War 3.
