Four mysterious artifacts.

Personally, no matter how many times I've played, I always found the Ring of the Ghost (or Spirit, depending on the translation) in the Hunter's Lodge in the Barrows location, and the Mirror of Medusa, as far as I remember, was in the dwarf mines in Bracada. It should be noted that amulets that grant immunity to petrification are also easy to find in shops, so this Mirror isn't that impressive.
Also, both of these artifacts "dropped" from monsters, rather than being found in chests.