3Dropped by
the beam runs on its own clock
Swinging faster does not give you more beams. The projectile has a separate cooldown that ignores your swing rate entirely, so what you get is a steady drip of ranged pokes layered over ordinary melee rather than a stream of them. Melee speed does still make each beam travel quicker once it leaves the blade, which helps you land them on something moving away.
where it wants to fight
A beam carries through one enemy and can then bounce off walls several times before it dies, so tunnels and enclosed arenas get far more out of it than open sky does. Fired down a corridor a single beam can rattle back and forth into the same group repeatedly. It also lights everything it passes, which makes it a decent way to see what is waiting further down a cave before you walk into it.
getting one
Armored Skeletons carry it and they are everywhere in the hardmode caverns, but the rate is low enough that casual mining will not produce one. Build a spawn area and grind it properly if you want it. Think of it as the hardmode answer to the Enchanted Sword, since it looks and behaves like a straight continuation of that weapon.