I was just doing the Stewartsville quest, when out of nowhere, some damn cloaked figure starts stabbing me. It nearly killed me. Is this supposed to happen, or what?

Not looking for the solution, I just want to know, because I just ran out as fast as I could.

Completely normal, if you screw up on the quest

I love how people on deathsense see you slain by "a cloaked figure." Like throwing a robe over Daevos' head while he pwns everyone masks his identity so the Celestians don't know what's going on.

Are you serious? I thought I had the quest pretty much correct to that point. I just thought I was supposed to beat him/her down or lead him/her to Gumboliah (which failed horribly, ending in my escaping with 30 health after bleeding).

You must be 100% correct, not "pretty much correct". If you aren't 100% certain or resort to guessing, then the murderer is going to hire an assassin to silence your investigation.

Part of the reason it's good to be a Faeling. If you just walk south, you can fly and wait for the bugger to bugger off so you can have another guess.. Or you could just run out of the village.. How far does the figure follow you?

Until he is dead. I've seen him kill newbies on the highway.

He doesn't have to be killed. He vanishes on his own after a couple of minutes

Ahah, that's what you think. I came back a few minutes later. Not more than thirty seconds after I re-entered the mansion, he popped out on me, right in front of Gumboliah.