So far, the closest I got is that your base mana regeneration gets a boost based on how much max mana you have.
Annie at level 7 should have 11.1 mana per 5. Mine had 20 per 5 (600 base mana).
I bought 3 saphire crystals, got to 1.2k mana, got 30 per 5 mana regeneration (doubled mana pool, boost increased 1.5x).
Mana regeneration from items did not seem to be impacted by this boost.
Huh, I was operating under the assumption that it worked like Chalice's passive like the Dominion and TT auras.
Initial badmath suggests that you get 1.5% of your max mana as X per 5 regen, or 0.3% of your max mana regenerated per second.
To quote a bad, 1 mana crystal gives you a free fairy charm.
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