Obsidian Pants

Obsidian armor is where a pre-Hardmode summoner build starts to work properly. The extra minion and the summon damage are fine on their own, but the set bonus is the reason to wear it: whips reach considerably further and swing faster. The reach is the part that changes how you fight. Whips exist to tag enemies so your minions pile onto them, and tagging from outside contact range is the difference between directing a fight and eating hits during it.

The evil material comes off the fight itself rather than the kill, so finishing the Eater of Worlds or the Brain of Cthulhu is not required. Chewing through a handful of Creepers or a stretch of worm segments and then walking away is enough. Pair that with Obsidian from a lava and water setup and Silk from your own cobweb runs and the set is available very early, for a class that normally has nothing decent to wear at this point.

Whip speed from the bonus multiplies with melee speed from other sources instead of adding to it, so gear like Feral Claws compounds rather than overlaps. Worth knowing early, because how fast you can swing a whip is the main limit on summoner damage at this stage of a run.

It gives no protection against being set on fire, despite what it is made of, so do not plan an Underworld trip around it. Shimmering a piece back returns the evil material your world uses, Shadow Scales in a Corruption world and Tissue Samples in a Crimson one. Drunk worlds swap between the two every in-game day, so which one you get out depends on when you drop the piece in.