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You know, it really is up to the DM to roleplay whether or not a deity is willing to revive a particular person from the dead. I'll admit, most DM's don't care to arbitrate cleric spells in terms of the cleric's deity, but this is the only real way to make sure either someone stays dead or is truly affected by negative game effects like ability loss and level drain.
Basically, just because you have alot of cleric spell power in the party, a DM should still determine if the god in question would grant ressurection type spells to raise a particular character. For example, a cleric of Tyr wants to ressurect a cleric of Bane because they are childhood friends. As a DM, I don't care what the cleric's level is, he or she is not obtaining any divine power from Tyr to raise a priest who worship's an enemy god.
Another example might be when a priest of Kelemvor tries to pray for a raise dead or ressurect to bring back a character who was killed during a non-essential adventure, aka non-world threatening. Kelemvor has no reason whatsoever to grant ressurection since death is a natural part of life, and even deaths caused by murder or plague must be allowed to occur without interference.
Cleric spells should involve some measure of roleplaying, especially powerful cleric spells such as True Ressurection or Miracle. As a DM, I have to arbitrate whether a god might grant a certain spell for a certain use based upon their philosophies. Otherwise, it is way too easy to avoid the negative consequences of being a hero.
The point I am getting at is that maybe Dartan figured the assassin would kill Jamison in such a way as to make sure he doesn't ever come back. If Jamison had been completely consumed by the creatures in the water, it would have taken far more powerful magic to bring him back than the party might possess at the moment. Heck, maybe the deities know something about Jamison the party does not and they may refuse to grant the magic needed to bring him back. If Jamison were evil, then most likely the dwarf cleric's god would not grant him divine spells to ressurect Jamison. Then only Vek could do it or a powerful wizard. Who knows, maybe Wee Jas might tell Vek that Jamison is dead and that she will not grant the power to revive him. It's all up to the DM.
Hopefully, Dartan was hoping none of the parties gods would grant the power to bring Jamison back after he had been wholly consumed. Although, it could be that not everyone puts such stringent requirements on cleric spells and bringing him back would be easy. Then, Dartan truly did just waste his time.
Either way, I see no problem with Dartan hiring an assassin. He is consumed with hate for Jamsion and is obsessed withing vengeance for what Jamison did wrong. This would be motivation enough to do whatever is needed to destroy Jamison.
