Kill 100 Kubrow during the extermination mission event 'Den of Kubrow', located on Earth, by the end of this weekend. You can *only* use melee weapons during the mission and you can replay it as many times as you need to.
Also, enter the promo code DENOFKUBROW at the warframe website for double credit reward weekend!
The reward for completing this event is the Wraith Strun which comes with one extra weapon slot and a catalyst