As the name suggests, you need to find One Million Mana lifetime (You don't need to be actively carrying that much).
It unlocked for me shortly after I finished leveling the entire skill tree.
This seems daunting but there are some methods:
Boss Farming
You can farm the redux bossfights. There are 2 that have good payouts and are quick:
The Mushroom fight, you can just spam Tornado Slash infront of the center mushroom and it will hit the two on the side. Though you will be popping potions a lot if you don't have the right skills unlocked, or the right weapons/runes to keep going longer.
The other one is the Homunculus fight, this is the redux of the fight that follows Holborn throwing you off the bridge. Bunch them up and do a quick Tornado slash, they should all die in a few seconds.
Each completion will get you about 3k mana.
The Mushroom fight costs 20k Gold and the Homunculus fight costs 15k Gold. You will recoup the cost plus some from the sellable trinkets.
Skeleton
This method is a lot better but can't be done until late game. The furthest map node to the bottom left in The Void is a long rectangle, closer to the right side of that node at the bottom is a red tree. If you keep passing infront of it over and over eventually a skeleton will spawn. It has more health than most and drops 10k Mana. Killing it should only be a few seconds. It spawns first time if you just loaded the instance, otherwise to get it to respawn you may need to run past the tree a good 10 or so times. Really good farm for Mana.