Hey there! When I did it, I hung around the divide (cosmodrome) for a few minutes until some Taken started appearing. Kill a few of them, and in the bottom left of your screen it should say something to the effect of 'A Taken lieutenant is plaguing the battlefield.' You have to find them and kill them. Sometimes they're Taken Minotaurs, other times Taken Captains, but they will never be more basic enemies such as Dregs or Goblins. Once you kill all of them (I believe there are always 3), a message will pop up saying that a Taken Champion is about to challenge you. Stick around the same area you just killed the lieutenants in, and you should see them; they'll have the yellow bar of health and everything. Note: you will need a fireteam for this, or hope that other Guardians will show up to help you, because they don't go down easy. Anyway, once you kill them, they should drop something that looks like those little white squares (not engrams. It'll look like House Banners, Black Wax Idols, etc.) and it should be your Mysterious Object, which I believe changes based on wherever you are. Hope this helps!