Ancient Shadow Helmet

Ancient Shadow armor is Shadow armor wearing the sprites it had before an early update redrew them. The stats line up piece for piece, and the two versions mix freely, so an ancient helmet over a modern chestplate and greaves still gives you the set bonus. Nothing about the old look is a downgrade. Wear whichever you prefer.

It drops from Eaters of Souls, which means the Corruption's most basic flying pest can hand you a complete set without you ever mining Demonite or fighting the Eater of Worlds. The odds on any single kill are dismal, but the enemy is weak and effectively endless, so a new character with a decent weapon can end up in top pre-Hardmode gear far earlier than the intended route allows. A Corrupt Statue also brings it within reach in a Crimson world, where the enemy that drops it does not otherwise exist.

Eaters of Souls fly straight at you without considering what is in the way, so two sloped surfaces funneling down into a lava pool do the work for you. Build it up in open air rather than dug into the ground, because Devourers will burrow through a surface setup and wreck the pattern. Once it is running you barely have to touch it.

The set bonus is a movement speed and acceleration ramp that builds as you keep running, closer to how Magiluminescence behaves, except this one keeps working while you are off the ground. The trail behind you is a line of faceless copies of your character along with a stream of purple bubbles. That effect is picky about slots. Putting the armor in social slots shows the trail without granting the bonus, mixing ancient and modern pieces grants the bonus but kills the trail, and anything else occupying a social armor slot removes it too.