Monk's Bushy Brow Bald Cap
Increases your max number of sentries by 1 and increases melee speed by 20%
3buying it
Monk armor comes from the Tavernkeep in exchange for Defender Medals, and he will not stock it until at least one mechanical boss is dead. Medals come out of the Old One's Army, so the real price is a run of the event rather than any material. The timing matters: you can be wearing this while the other two mechanical bosses are still alive.
what the bonus changes
The set makes Lightning Aura sentries tick faster and gives them a fixed chance to land critical hits, which sentries otherwise never do. Summon critical chance is rolled as its own separate event instead of being added to your normal crit chance, so running the set with a Morning Star or Kaleidoscope produces a higher real crit rate than adding the two figures together would suggest.
melee speed and whips
The other half of the set is a melee package weighted toward attack speed rather than raw damage. Whips scale off melee speed and ignore melee damage bonuses entirely, so a whip user gets more out of this than the tooltips imply. That is also why the cap is worth wearing with Spider armor in place of the Spider Mask: you keep the summon bonuses on the body pieces and take the speed on your head, and the result behaves like a Hardmode take on Obsidian armor.
monk or squire
Squire armor is the other melee option on the same counter, carrying more defense, faster regeneration and a bigger critical strike bonus. Monk wins on speed, which pulls ahead against low defense enemies and against everything once a whip is involved. Squire is the safer set to wear. Monk is the faster one to fight in.
where the auras fall down
Lightning Auras do not move. Against Plantera and the mechanical bosses that come to you, ringing your arena with several of them makes the fight far easier, and the set competes well with Hallowed armor at that stage unless you want the Holy Protection dodge instead. Against The Twins and Skeletron Prime, which spend the fight circling at a distance, stationary auras do nothing, and the Queen Spider is only a partial answer since its turrets need ground under them. Shinobi Infiltrator armor is the direct upgrade when you can afford it.