It is actually possible to get 7 Shadows of Doom from one site. If you kill 3 of them, and then wait with the fourth left alive, another 3 will spawn. Once you kill 4 the area will be disabled and no more adds will spawn, but the three channelers with stay allowing you to interrupt them and spawn a Shadow. All of these shadows will drop 30 runes. I did this on Monday, so it is possible that this has been changed.

Another thing worth mentioning is that once the channelers are up the spawned mobs are of a higher quality and seem to be more likely to drop runes.

- Souca -