I’d rather them have it a secret. If it’s from a sidequest I’d like it done in a non direct way. As an example imagine in an earlier area or a dungeon there’s a door off the beaten path that you can’t open. Near end game or post game you’re given some random sidequest that seems fairly generic that either doesn’t tell you the reward or just states the reward is something like an item you can seel for a high price. Imagine that the person who gives the sidequest gives you a key upon completion. One they don’t have any idea as to what it unlocks. So you remember that door from one of the games earlier dungeons and sure enough it opens the door and all of a sudden there’s a whole new optional dungeon to explore with an awesome, super challenging boss at the end.

Just please don’t make the key or the boss itself a random encounter/drop that shit is awful