What's the reason? For the darkness, I mean...

[Frost]

But i am pretty sure the monster can't see you. It just walks away and vanish.

If you run right up to it, it will certainly see you. And kill you.

And yes, they are Alexander's servants.

As for the darkness, I don't think there is a real reason explained in-game. I think it's just to creep the player out.