When an item is auto-added to the discoveries list by you seeing it for the first time, also mark on the list which level you first saw it.

Vorpal Blade should always decapitate Medusa. When this happens, it produces a “Medusa head” comestible, which is a unique, unwishable item which could either confer stoning resistance when carried or, like in the myth, be used to stone monsters at range.

An alternative, to make it more broadly accessible, is to allow any bladed weapon used in melee to decapitate Medusa, and only her, on a critical hit (while Vorpal Blade will decapitate on any hit that lands).