Harbinger's Echo is an Exotic Sparrow obtained by shooting out all 40 Corrupted Eggs in the Dreaming City with the Exotic Combat Bow which is called the Wish-Ender.
Obtain
To obtain that that Sparrow you need to go to the Dreaming City.It takes a long time to obtain because some eggs can be find all across the city then you can find some in the Last Wish Raid.More eggs can be find in the Ascendant Challanges but it's important to know it takes several weeks because each week contains a different challenge.