Dream,
I believe that the dark priests would create a very interesting imbalance of power because:
1. They both share devotion, hence they would both be opposed the priests privately, yet united with the priests against other city/guilds
2. Dark priests would not suit Ashtan, because Ashtan guilds (infernals in particular) do their very best to weaken devotion, a skillset of their enemies, the Priests and Paladins.
3. Therefore, the Dark Priests would be aligned to the Light Priests against Those who seek to weaken devotion, yet also opposed to Light priests by their very nature.

Perhaps there should simply be a separate "sect" of priests, partly aligned to the guild, partly ..erm..not aligned. I'm not sure how that could work, but The idea would be to honour darkness, and not evil...and I dont mean Twilight as opposed to Sartan, I used those terms for want of better one only.
That is the only problem I see with the whole idea...i.e. this mixed loyalty business.
Also, how would a dark priest fight a light priest at the location of a shrine?
Would it be a rush to see who could do the rites quickest? Could a shrine do multiple rites at once? Can a shrine in fact even be used against a priest?
I would appreciate some feedback from a priest on this, if you could.
Caled

Penned by my hand on the 23rd of Daedalan, in the year 250 AF.