Successfully complete the indicated task to unlock the corresponding mod:

Active
  Invisibility: Get 300 kills with Jigsaw in Multiplayer mode. 
  Invulnerability: Get 300 kills with Silver Sable in Multiplayer mode. 
  Regenerator: Get 225 kills with Barracuda in Multiplayer mode. 
  Sprint for Life: Successfully complete Story mode Level 1. 
  Weapon Upgrade: Successfully complete Story mode Level 3. 

Passive
  Berserk: Get 250 kills using the Rhino in Multiplayer mode. 
  Blood Gain: Get 150 kills using the CMG in Multiplayer mode. 
  Link Feeder: Get 300 kills using the Enforcer M8 in Multiplayer mode. 
  Motion Sensor: Get 175 kills with Microchip in Multiplayer mode. 
  Rampage: Get 275 headshots in Multiplayer mode. 
  Thick Plates: Successfully complete Story mode Level 2.