As to the background question, there is a bit of information posted earlier here:
Background Info stuff
If that helps a little bit, cool. In addition to that, I would say that King Sigfried (See information thread re: post on Barghost) has been advised by the priests of the temple of Pelor in Hipposus to send in a reconnaissance squad to estimate the threat level of the minions in Quehalost. The purpose of the squad is to enter, observe, and destroy anything and everything that is within their capability - so long as it is evil.
Religiously, it is being touted as Pelor's becon of light shinning into the dark side of the land.
Politically, it is Barghost coming to the aid of its neighboring lands and bolstering them in the fight.
Militarily, it is an advanced party sent in to gather information for a possible massive military action against the evil infiltrating the land.
Without giving away too many specifics, that is the jist.
Of course, this is not a "typical" scout party beause the dangerous level of fighting will require much more than just stealth. I don't want people to get the idea that this will be a "hide-and-seek" type game. More like a "search-and-destoy." I hope that is music to everyone's ears. There will be brief periods of "Oh look, a good (or neutral) person struggling against the system and trying to make a difference." There will be periods of "sure, we can help protect your caravan as you try to deliver medical supplies." (No, not that literal example.

) But for the most part it will be, "What ... a demon? SMITE!" or even "What ... a wight? SMITE!" How that happens, of course, is up to the party. It could be full of "going in with guns blazing" type encounters or it could be "scout 'em out and pick off the stragglers" type encounters. That depends on the party's style and gifts. My guess is probably a healthy dose of both types of approaches given the circumstances at hand.
But ... I digress. I do that alot. To make a long story short - the party is being summoned to infiltrate Quehalost. Nobody really knows what they will find and how bad it is. Demons, undead, and even devils are reported to have crossed from Quehalost into Tongra and Fenneress - although not in the same places and not necessarily woking together. The party is going to find out information and do what damage they can. [There might be a few other caveats added along the way, but this is the main jist.] I suppose in many respects it is not unlike a US Navy Seal mission done D&D style. Go in, kick butt, collect info, and if you value your life don't get captured or you're coming back against the enemy as a wight ... or vampire ... or tortured soul ...
Okay, I am now getting carried away. I hope the above direction of the game is intriguing to the interested players. I have tried to paint an exciting approach to the Good vs. Evil clash of wills.
Oh .. yeah. You asked about the Slayier of Domiel PrC. This game has really gotten me thinking about tweaking the cosmology of the typical Greyhawk world. Not eliminating things, but regrouping the deities into "families." If you don't mind the possibility of being a "Slayer of Pelor" or some other related deity (perhaps even Domiel, but unlikely) then certainly the class is approved for future use. Essentially, nothing about the class would change except the deity venerated. If you are really into Domiel for some reason, then lets talk. If you've got a specific reason for wanting to pick up worship of Domiel later on in the game, I am certainly willing to listen.