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Yes, at one time there was such a thing as mana burn in League of Legends. Wit's End was/is an anti-AP item, but the mechanics of the weapon have changed drastically in recent times. Currently Wit's End stacks MR the more you attack. Originally the item burned off 40 mana per attack on the target. In theory you would target a mage and burn off their mana pool. In practice the Wit's End would be used to burn off all of the tank's mana pool since tanks don't stack mana. Needless to say, this mechanic was found to be a little OP. The item was originally intended to be anti-mage, but it instead was used to starve tanks of mana. Riot decided that this needed to end, and then the new Wit's End was born.