“Kill off the end of the Vex as Praedyth saw it.”
Praedyth welcomes you to the end of the Vex, their "immutable" future. There are a number of Taken Goblins around that you need to defeat. They are the warm-up here. Just after them you will encounter more Taken Vex with Hobgoblins and some Taken Minotaurs. Get through the initial waves and the boss will appear: a Blighted Descendant.
Here, again, it is very important not to die. Having just one other person can make a big difference. There is a fairly constant stream of adds in this fight. Use them to refill ammunition if you get low and recharge your Super. Focus on its head. While it is invisible the whole time, save when it has taken a lot of damage, it can still be damaged normally.