Here is a 2019 solution, because the game change a lot.
In the strikes section you will see 4 missions what you can do, YOU HAVE TO SELECT THE ONLY SAID "NIGHTFALL". NIGHTFALL THE ORDEAL DOESN´T WORK IF IS NOT A FORSAKEN STRIKE
You can do one of three are in these section, the strikes of the Forsaken DLC is:
Warden of Nothing
The Hollowed Lair
You have to do the Forsaken nightfall to unlock the achievement, there is a chance per week to see one of the above in the nightfall section
At this point 10/22/2019 Is the corrupted nightfall up!