Ethereal Core

The Ethereal Core is a craftable permanent power-up item that can only be consumed once, and upon consumption, increases the player's maximum mana by 50. There are 2 other items that function identically to this: the Comet Shard and the Phantom Heart. It can only be used once, and after that the player must use one of the remaining items to increase their maximum mana. With all of these, the player's maximum mana is capped at 350.