First off, spoilers for Darkfall cave ahead. This being the spoiler forum, I wouldn't think it necessary, but ... internet.
If you haven't found/been through Darkfall Cave - stop reading and do it!
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No, seriously - it's up north - southwest a bit from Mor Khazgur. Go.
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Now then, tiny cave out on the edge of nowhere I found while traveling to the Orc Hold Mor Khazgur. I am the consummate explorer in RPG's, but if this thing didn't show up on my compass or the map, I probably would have walked right by/over it. There may be a quest to send you here (I can see how it would make sense), but I didn't have one.
It is pitch black. You are likely to be eaten by a grue.
That is exactly the feeling I had going into this place (and also exactly how long I've been playing these things). I head down, bow drawn. I even thought about that as I went down the long entry tunnel: the odds of whatever lives here not being actively hostile are slim, even though we weren't near any forsworn camps or whatever. Opened into a mining type thing with a stream running through it.
Explore a bit, mine a node, looks like about done when the wooden bridge collapses under me - swept away by the current, and I can see glimpses of spiders as I'm pulled down over some falls. It just got real. Fight off the spiders, it's still dark, and I can't get back up the way I came. This obviously was meant to play out like that so I didn't even think about reloading. I'm svcked in at this point.
Stealthily go around, kill a few more of the beasties (fortunately one of my companions hits with ice so they glow a little bit or I'd really have been useless) and find a little room with a campfire. Wait, there's a dead chick. Spiders must have got to her, but she's not in a coccoon... hmmmm.
Read a note to her sister: Sorry I'm going to die, blah blah blah, thought we could live next to these creatures (what? thought you could live next to spiders?). These trolls would understand us... *scrrch* back up - Trolls? I didn't see anything about trolls! Crap.
Another tunnel deeper down (this is obviously not the way I'd like to be going). Cool creepy old shrine place. Big shapes moving in the shadows. Snipe, Snipe. Dead trolls. Find a shrine. Maybe that's why the women wanted to live here? Or thought these trolls would be different? Search around, some little stuff. Click the shrine - it still works.
Now all I have to do is get out (where's my recall spell?). Search, search, walk around the circle, find a secret door (that's how the trolls surprised the sister - orcs and Bilbo Baggins style), search, search some more, check the map again, up river, down river, finally find a tunnel leading up, hidden door, another tunnel - hey this looks familiar. I see daylight ahead.
Yes! back safe in Skyrim. (quick glance around for ironic pissed off giant - nope, I'm good).
THAT IS HOW YOU DO IT! I really hope I find more things like this and I didn't just go through the best part of the game, but if even if that was - I will remember that cave for a long, long time.