Don't forget the permadeath mechanic if these challenges were too difficult it would be a real pain. Also please don't add any timed challenges I hate time limits in games. Much prefer puzzle or strategic challenges.
Perhaps if combined with another proposed mechanic, the 'rare' gems. Or even if combined with the 'enchantments', lets say, a boss will drop an enchantment which can make a gem into a 'rare' if you beat the boss in less than a minute?
It would just give some occasional 'seat-of-the-pants' sensation in a game which is otherwise quite meandering. (in a good way
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