Lunar Flare

The flares fall from above and travel straight through terrain until they reach the spot you clicked, then go off. You can therefore hit something on the far side of a ceiling as long as your cursor is on it, which is rare for a magic weapon and makes this strong in the cramped parts of a world where most spells are useless.

Each individual flare can only damage a given target once, so there is no piercing chain out of a single projectile. All three can still land on the same enemy, and that is where the damage against a boss comes from. The catch is that a flare which clips something on the way down detonates early and never reaches the target you wanted.

The blast radius is forgiving enough that precision is not required. Point at the ground under a crowd and let the spread work. If you used the Meteor Staff earlier in a run, this is the same weapon with the numbers moved up, and it wants the same habits.