It took a lot of coil runs for me to get the Adhortative to drop. Been told it can also drop from Rivens lair but as thats shorter and has less drops overall - it would likely be the worse option to run for the weapon unless the team you're with are slow with the coil. If you're focusing the undying row engram at Riven, it wont work, the weapon pool for that engram only contains the reprised undying weapons you already have.