Forgive the cryptic title, though the post is no less -as the title suggests- mysterious. The last occurrence of the problem I'm about to describe was in Solitude. A penitus occulatus agent said something distasteful to me so I impaled him with an ice spear, the guards didn't seem to care. I walked away towards the gate closest to the temple of the divines, that leads to proudspire manor and just as I passed the courtyard where I killed Vittoria Vicci at her wedding. I saw a figure, completely shrouded in black, like the visual effect on your character when you have the top sneak perk and you sneak during combat. He was as tall as me, (my character's an Altmer) and was there for a second then he disappeared into a wall and before you insult my intelligence, no he was not my shadow lol. He didn't mirror me. I saw him again when I was fighting an ancient dragon in Falkreath he was standing in the middle of the big graveyard there and once more in the midden dark of the College of winterhold.

Everytime this...whatever it is appears my game gets very laggy and the music stops and he just stares at me, he appears to be wearing nothing at least that's what I can tell by the shape of him around his legs I can make out what appears to be a loin cloth, also backing up this claim and he is covered head to toe in thick black, he's like a living shadow, that appears and disappears at whim, just staring, always watching, never engaging.

Does ANYONE have any idea what this thing is? I have NO explanation, but it creeps the hell out of me. Sorry I couldn't screencap, I'm on XBOX so I don't have that option and this always happens too fast to pull out my Ipod cam lol.

Maybe it is a reflection on your TV from the bogeyman that is standing behind you as you play Skryim (yeah, think about that the next time you play with the lights off !)

Seriously though, have you tried getting close to him in order to see if this 'thing' has a name? Can you paralyse him/freeze him with a dragon shout, and look at his inventory? Maybe this will give you a clue as to what it is.

The lag could be a symptom of this object going wrong with the collision detection. 'Havok' (the physics engine that Skyrim uses) can consume large amounts of processor time if a particular object goes wrong (hence the slowdown), as it attempts to resolve a collision with any part of the background.

Edit: possibly, it is a corrupted NPC, which has no lighting attached to it, which is why it appears black.

I considered this and it just appeared there I got close to it and now I'm even MORE confused. This time it appeared right next to me, it's "name" is hero/01110010 01100101 01100001 01110000 01100101 01110010/ahfa=A and it made that sound that reanimated corpses make, maybe it's a glitched one of those.

but yeah I got no clue hahaha corrupted NPC is very possible, I'll do a headcount in each of the holds, I remember the faces and names of all those I've killed myself so it should work lol.

It always appears too quickly to be attacked, even with a dragonshout, see even if you pause the game or bring up the hotkey menu, it still disappears. It's all very very odd.