Last Prism

It opens as a spread of separate beams that circle and tighten, and after about three and a half seconds they merge into one column that hits vastly harder than the spread did. Each beam in the spread is doing full weapon damage the whole time, so the ramp is not dead time, but the merged column is the reason to hold. The catch is that the mana cost climbs steeply as the beams converge, so the weapon effectively charges itself by burning your pool. With no support you get five or six seconds out of a full bar and then you are holding an empty stick.

Unfocused firing is dramatically cheaper. Against a room of small enemies, or anything wide enough that the whole spread lands on it, short repeated holds do the job for a fraction of the cost and you never pay for the expensive end of the ramp. Save the merged column for a single target with enough health to justify the mana. The aim is worth knowing about too: the beams sweep toward your cursor rather than snapping to it, so anything that circles you outruns your own weapon.

Drinking through the fight works, but mana sickness scales its damage penalty against how fast you are spending, and there is a hard cutoff. Burn above the line and the debuff takes an enormous bite out of your output. Get under it and the penalty shrinks to a small fraction of that. Since this weapon charges you many times a second, small percentage reductions matter more here than on anything else, and the goal is to push its per hit cost down to a couple of points. That is why the cheaper casting prefix competes with the crit prefix on this particular weapon: it ignores attack speed entirely, so the choice comes down to flat crit against the reduction that carries you past the sickness threshold. A Mana Flower plus reduction headgear and a caster robe stacks far enough that potions become safe to drink freely.

Mana stars carry no penalty at all, so any way of generating them beats drinking. A statue farm beside your arena killing weak enemies on a timer, with a Celestial Magnet or an upgrade of it pulling the stars in, will feed the beam for as long as the farm runs. The other trick converts damage into mana: stand on spikes with Magic Cuffs or Celestial Cuffs equipped and you regain mana faster than the spikes take health, especially with regeneration accessories on. An Adhesive Bandage or one of its upgrades cancels the bleeding they inflict. All of that requires standing still, which rules out most bosses but suits the pillars and the Old One's Army well.

The beam cuts out after a full minute of unbroken firing and, annoyingly, keeps draining mana while it is off, so do not lean on the button through a lull. It cannot pass through terrain, though it clips through visually and its hitbox reaches slightly past a thin wall, which is a curiosity rather than a tactic. The Spectre Hood pairs badly with the merged column, because the hits come so fast that the healing orbs often fail to appear at all.

Open sky and a target that holds still. That is the whole answer. In a cramped arena the beams eat walls and you get nothing out of them, and the Lunar Flare off the same boss covers those spaces properly. Given an arena and something feeding it mana, this still puts out the highest magic damage in the game, enough to end a Moon Lord rematch well inside its own one minute cutoff and to push a Pumpkin Moon to scores most weapons cannot approach. Only the Zenith beats it outright now.

Name your character after one of the game's developers or testers and the beams come out in a different color set. Naming yourself Random rolls the palette instead. The same applies to most of the weapons the Empress of Light drops.