The jar of eyes is a rare drop from Sarachnis found in the Forthos Dungeon.
The jar can be used on a boss lair display in a player's Achievement Gallery to have a display of Sarachnis. Doing so requires killing it at least once.
Like most items obtained in the Forthos Dungeon, it can be used on Brother Aimeri for some additional dialogue.
For an exhaustive list of all known sources for this item, see here
- The jar of eyes is the only boss jar that appears to have solid items inside of it, as opposed to other jars which contain a liquid-like substance of a singular colour.