The Ioun Stone of Mastery is a legendary item and so far as I know the only feature outside normal level progression that offers an increase in proficiency.
I would like to use one in my game but I am wondering about the power level. It looks to my like a combination of items that are otherwise much lower rarity, i.e. part of a Stone of Good Luck plus part of a +1 weapon plus a Wand of the War Mage.
Is that combination significantly better than the sum of its parts? Or is the legendary more due to a "coolness factor"?
In case this is relevant:
- I never play tier 4, almost exclusively tiers 2 and 3.
- I handle balance rather loosely.