Celtic Heroes

I have initiated the Darkness Released quest in Dustwither Catacombs. I am supposed to kill Disciple Ducarius, Disciple Kiara, and Avatar of Donn, whom I am assuming are all in the Catacombs. However, I have yet yet to see any of them spawn, or alert me with the double swords image on my map, and I have spent quite a bit of time on several occasions waiting for them. Can anyone else fill me in on what the issue might be?

none, you just gonna wait. and possibly kill their minions (elder priest) or higher spawns (arch disciple or invocation).
there is one boss of this quest in every one of the three roo(ms at the top of your map

Yes you just have to wait. But in this time you can farm in catacombs