I found darkfall cave while randomly wandering, around the Markath area, and then entered it later to gather falmer blood for Sigmus Septimus. I only found out very recently that there was a whole string of quests related to the cave, none of which I have active or accessible in anyway, except the base Dawnguard quest that won't help me. I'm either an idiot or actually stuck because I can't find a way out and I have no prior saves to load from. I've been stuck going in circles for hours, Ive been through the dead bretons camp, where the frostbite spiders are (were), defeated the trolls and been to the area with the shrine, so feel free to use these locations as reference points. as far as I know I've been through every possible place I can without making it outside. Am I missing something? If anyone can give me some help, it would be greatly appreciated.
I assume you've used the map, right? If you leave the shrine area, the dead Breton is in the direction of the exit. Work around up there until you find it. Otherwise, come back here, and I'll try to help more.