Potion to make you phantom-like?

[Usulama]

You can get a quest somewhere (can't remember where) that prompts you to investigate a tomb. There is a guy hiding there who looks like a ghost but becomes normal when you kill him, and by reading his diary you learn that he makes these potions that make him look like a phantom so he can scare away the locals.

He has one of those potions on him, and another one on a shelf nearby. I've played a LOT of skyrim and never seen one of those before?
So now I have 2, but I don't dare use any because I don't know if I can get more in the game, or make any myself.
They aren't worth much, and most rare things are.

 

[AS88]

http://www.uesp.net/wiki/Skyrim:Lifting_the_Shroud

Here's the one. The potions aren't craftable, but according to the notes more will spawn if you wait long enough. They don't really benefit you in any way, either, no special dialogue or anything.