welp was up all night ranching slimes again. getting into postgame stuff now

I'm waiting to get the purple treasure unlocking upgrade but I'm not sure what the condition is. I thought it would be number of treasures unlocked but now I have a sneaking suspicion I need to do some number of trades with a specific NPC. I don't know which one it would be though since I haven't been paying enough attention to the text in this game (though I can always go back to read it all)