If you read the description I gave above, you will note that the seven priests appear to be engaged in a ritual of some sort, and a rather flashy one at that. However, you do not see the supposed leader of the Fane, High Wyrmlord Azarr Kul (said to be a half-blue dragon, so he has a pretty distinctive appearance).
You can infer from this that whatever they are doing in there may well be tied to the ritual, but is not the heart of the ritual itself. If you were to interrupt them (say, by slaughtering most or all of them) it might cause a delay before the ritual is complete, at least until they are replaced, which could buy you a few days' time. On the other hand, if you push too hard trying to take them out, it could end up with most or all of you dead, in which case it could well be that the ritual will be completed before Brindol can recruit more people to go in and finish cleansing the temple. Decisions decisions!
Trellara, you in particular should be careful, since you are the primary means of escape for the majority of the party. If you fall, they'll be either stuck in the fane, or forced to go out the front entrance and likely deal with the dragons short of allies and spells. Note that the enemies may well be aware of this fact, as they likely have received intel on your group's abilities from your fighting in Brindol and your prior forays into the Fane. It is almost certain they are aware that you are magically transporting yourselves in and out of the Fane.




('cuz Yuku's lame)