If you're struggling to find powerful fallen to kill with drang after speaking to Tyra, then there is an easy exploit. If you speak to failsafe and reactivate the adventure "Exodus Siege". It's gonna put it back on the map for you and you can activate it like normal. Once you activate it you are told to kill a cloaked fallen that is right nearby. This vandal counts toward killing powerful fallen with drang. Simply kill the fallen, abandon the adventure, and pick it right back up again. Enjoy!