The quest automatically prompted as "failed" the moment I wanted to begin the ritual - so the possibility to obtain the 18th dagger only after the ritual made no sense :(

You can Finish this quest now, I think they patched it, you can find all 20 daggers before the ritual, and then once you finish the ritual and fight the last undead that are killing the "cultists" you can get the last one. You do need to be thorough with your search for the first 20 daggers though, some of them are in places you wouldn't normally go into :p

the last dagger from when you go outside and kill the tigers and undead... make sure you hand in right after that, dont "talk to the prisoner" first.

I know this is a long shot since there's not much activity here but I am stuck on this quest and cant find answers elsewhere. Am I doing something wrong or Is there some trick to get Ajb,Afjine,Gwei, and Fiahn to drop? The only daggers I get are the others listed to drop from those wights and those 4 are the only ones I keep missing. I have tried just about everything and for whatever reason I simply can't find them.

thanks for the tip guys. 20 daggers and then blow the quest off until it gives you the last one.

I received 2 of the "Orren the Bear" daggers, and I did not receive the "Inbalta the Fire-Haired" dagger. I have 21 daggers, but as I said, I have a duplicate of one and missing another one. The quest won't finish.

Similar to the post right before me, it's possible to receive two copies of the Orren the Bear dagger. It seems at some point the devs made a change so that the dagger was spawnable before the skeleton encounter from continuing on the ascension quest. It spawns from a perception check at the end of the middle ramp into the middle basin area; almost exactly where your party is placed as part of the quest encounter. In fact the Orren the Bear dagger from the skeleton encounter is NOT the one you need in order to turn in the quest. It will be taken up if it's in your inventory when turning in the quest though.