Celestial Cuffs

Damage taken refunds mana equal to the raw incoming hit, worked out before your defense and damage reduction cut it down. That inverts the usual logic: a heavily armored mage takes a scratch and gets a large slice of the bar back for it. Falling counts as a hit for this purpose. Drowning and debuff ticks do not.

Because the refund tracks the attack rather than what actually lands, self damage is a reliable mana faucet once your defense is respectable. Spikes, explosives and Exploding Bullets all work. The tidiest version is a Slime Statue wired to a lever beside your arena, where you can top the bar up between waves for almost no health.

The other half is a wide vacuum for mana stars, and in play that is the part you notice. During an event where things die constantly you simply stop breaking off to collect. It gives you nothing extra next to a Magnet Flower, a Celestial Magnet or a Celestial Emblem, and the mana refund does not stack with Magic Cuffs, so run one and free the slot.

Every component is reachable that early, which is unusual for something this useful. In a Crimson world the Band of Starpower in the tree is the obstacle, since it normally comes out of Corruption orbs. Building one in a Graveyard works, using a Mana Crystal plus a Panic Necklace, and so does fishing Defiled Crates in a small patch of artificial Corruption.

Anything with a heavy mana bill, the Last Prism most of all. Compared to a Mana Flower it sidesteps mana sickness entirely, which matters when the damage penalty would cost you more than the drinking saves. Paired with the Spectre hood it closes a loop: damage dealt heals you, damage taken refills mana, and the mana pays for more damage.