Skyrim is, as you may be aware, a pretty massive game. Without even accounting for the cartoonishly-high number of mods out there, the fifth Elder Scrolls game is basically endless. So, naturally, with that much space to toy around with, Bethesda left a bunch of weird little Easter eggs hidden throughout - and a lot of non-hidden stuff that you just never noticed because the game is so stupidly big that lots of people who plunked over 100 hours in never even finished the main storyline.
Thought you had faced down every Dragon Priest, Legendary Dragon, and Named Monster that this slice of Tamriel has to offer? Guess what - probably not.
Well, honestly, you probably DIDN'T think you had, because you're aware of just how massive Skyrim is. So it should come as no surprise that there are (at least) 3 hidden bosses strewn about Skyrim - although you'll need the DLC to face them all. If you only have the vanilla Skyrim experience, then you can at least take on one of these bad boys.
The first requires you to head down to everyone's favorite surreal Dwemer ruin, Blackreach. There's an enormous orb towards the center of the underground cave that's pretty hard to miss - just stare into it, give it the ol' Unrelenting Force shout, and BOOM! Now you're fighting an underground dragon known as Vulthuryol. It's not a particularly difficult dragon, but it's at least tough to find. Plus, there's a nearby giant, which (assuming you didn't already kill it) pretty much guarantees a dragon/giant battle.
The second one requires the Dragonborn DLC, which you should probably have anyways because it LETS YOU RIDE A DRAGON (fair warning: riding a dragon is way less cool than you'd think). But if you do have it, you'll get to fight a boss that's a throwback to the good ol' days of Morrowmind (well, Morrowind's Bloodmoon expansion): the ghost of the Frost Giant king Karstaag. To summon him, you must retrieve his skull from the Glacial Cave, placed on the throne at Castle Karstaag Ruins, and you'll get to fight his spectral form.
It's a tough fight, but the reward is pretty neat - you will gain the ability to summon the ghost of Karstaag, but only THREE times ever, and only when outdoors.
And the third secret boss is The Reaper, only available in the Dawnguard DLC. First, you need to head to the Soul Cairn, and from there head in to the Reaper's Lair - place three Reaper Gem Fragments on the altar, and get ready to fight THE GRIM REAPER.
Sure, the Thieves' Guild is something of a mess in Skyrim (at least, compared to the glory days of Oblivion) - they're reduced to living in the sewer like some friggin' Ninja Turtles, there are traitors running amok, and YOU - a completely new member who spends a lot of his or her time yelling at dragons - are pretty much their only hope in the world. But - they're still the Thieves' Guild, and have a bit more going on than you might notice. For instance, check out pretty much any building in Skyrim, and you'll see that the Thieves' Guild have left their mark there.
Yes, ANY building.
Well, okay, not ANY building, but definitely a whole bunch. The markings are called Shadowmarks, and basically work similar to the way Hobo Code markings would work. And in case you're not familiar with the Hobo Code (either due to not being a hobo in the 1930s, or not seeing that one episode of Mad Men), here's basically how it works: there are a bunch of symbols that members of the Thieves' Guild can leave on a building to communicate things about the building or its owner to other members, but would be indecipherable to anyone else. There are symbols that indicate the owners are members of the Guild, or enemies to the Guild, or that the store has already been plundered and there's nothing valuable inside, or a bunch of other things.
Whiterun and Riften (no surprise there) have the most Shadowmarks, but there are a number of them strewn about the rest of Snowy Dragon Land USA.
As is common with caves in Skyrim, the cave at White River Watch is teeming with bandits - but there's one bandit who's not much of a threat: Ulfr the Blind. His non-threatening nature should be pretttttttttty obvious (hint: he is NOT the fantasy version of Matt Murdock). But what's really great about Ulfr is that - despite being blind - he's something of a bookworm. And what makes that EVEN better is that THIS is Ulfr's book: