Destiny Eris Morn Bounty GATHER THEIR DUST
You need to buy it from Xur for 1 Strange Coin. Eris Morn will then give you quest steps that you need to complete. http://youtu.be/dbW87zUagZ8
You need to equip the a Fusion Rifle with Solar damage. The newly added MURMUR from The Dark Below DLC comes in very handy.
The first one is “Gather Their Dust” for which you will need to kill 25 Thrall and collect their Embers of the Lesser.
Probably the fastest and easiest way to get them is to start the mission “The Dark Beyond” on the Moon. Deploy your ghost in front of the gate and kill all Thrall that are coming out.
After you kill the first bunch let yourself die without picking up any of the embers. They will still be there after you respawn. Now you can kill the Thrall again and collect your 25 Embers of the Lesser.