True Excalibur

The rework changed what this sword is. It no longer throws a beam. Instead the swing drags a wide field of light through the air that reaches well past the blade and wraps around behind you, so you hit things you are not facing. Against a few enemies bunched together that is a lot of free damage. Against one target it is just a sword with a nice glow.

Plantera is the fight it was built for. The tentacles she spawns die to the aura without you aiming at them, which keeps the arena clear while you concentrate on the boss itself. Knockback always shoves enemies away from you no matter which way you swung, so it doubles as a panic button when something gets on top of you.

It is a stepping stone. The destination is the Terra Blade, and holding this plus Excalibur puts you most of the way there, so do not get attached to it.