I'll have to read over the Fallen. Not sure how well Yuan-ti would fit with the fallen, they are monstrous humanoids(rather than Outsiders-Demons & Devils). Might go the suggested route in the Yuan-Ti class post and eventually take cleric. Progression would most likely be Yuan Ti 7(levels 1 through 7)/Cleric 13(From Level 8 on up). Though with the spellcasting penalities for not worshiping The Dark God, it might not be a good idea. (I know he can follow an ideal, but I perfer to have my clerics worship deities.) And also the Yuan-Ti would probably be Chaotic Evil(Smart). But it's seeming as though Yuan-Ti might not be a good fit for this campaign, but still open to the idea. How would a Yuan-ti cleric work in this world.
And if not yuan-ti I still have plenty of ideas.
Human Cleric of The Dark God
Sun Elf Generalist, Liscenced Magi,
Sniper of some type(Wood Elf, older human(grizzled military vet) ).
Dread Necromancer.
Maybe even the cliched gnome that just loves fire.
But if we could make a Yuan-Ti Abomination Cleric work, I think that'd be fairly cool.