When you reach earth after visiting Lostlight, speak to Uriel. She wants you to track down and kill 10 tortured angels on Earth. This side-quest can be completed as you make your way through the story level on Earth. Follow the main walkthrough for Act 3 to find all of them.
After they are all dead, head to Uriel to complete the quest and be rewarded with a legendary secondary weapon - Sunder.