This is aimed at you game of thrones fans. Basically, I'm looking for a faction that is as similar as possible to the Nights Watch. My character is getting to the end of his fighting days as the leader of the Thieves Guild, and as a fitting end, I want him to "take the black" per se. Anyone have any suggestions on which guild I should join?

The Dawnguard? I don't really like Game of Thrones, but Nights Watch essentially means protecting against 'bad things', right? Dawnguard serves a similar purpose. Plus they'll come looking for you, so you won't need to break any roleplay to join.

Vigil of Stendar, but you can't really join them.

I haven't watched Got, but they sound similar to the Dawnguard.

I'm in agreement about the Dawnguard seeming to be the closest thing you can join. Perhaps the Vigilants would be closer in that sense, since the Dawnguard is very vampire specific, but you can't join them.

Any guilds that deal with the forsworn on a regular basis? Because the Nights Watch are all about killing wildlings, which are essentially the forsworn

Not really, unless you count any mercenary and guard working in the Reach.

nothing seems to come to mind in regards to the Night's Watch and anything in Skyrim.