Meowmere

Each swing lobs a cat head that sags as it flies and keeps going after it hits something. When it strikes a surface it rebounds with a loud meow and detonates in a rainbow burst, and each successive bounce widens the next burst. The last one covers noticeably more ground than the first.

In open sky most of that potential is thrown away, because the projectile arcs off and never touches a wall. Put surfaces near it and the same swing becomes a chain of explosions filling the whole space. The dungeon is where it looks absurd. For an open arena boss fight a flatter shooting weapon puts out more.

Selenians during the lunar events knock the cat heads straight back at you, and at your own damage level that is lethal. Keep your distance or swap weapons while they are on screen.

The projectile is a Nyan Cat reference, and the sword carries the highest listed hit of any non explosive weapon in the game. That is not the same as the highest sustained damage, and the Terrarian, Star Wrath and Zenith all beat it over a long fight. It goes into the Zenith anyway, so hold onto it.