Mechanical Skull

It only works after dark, and that is the real planning problem. A mechanical boss fight runs long, and triggering one late in the night can lose it to sunrise with the boss most of the way down. Summon right as night starts and give yourself the whole window.

Any hardmode enemy that drops coins can hand you one at very long odds, so the first one you see will probably arrive in the middle of something else entirely. That is a gift, not a prompt. Bank it until the arena and the gear are ready instead of throwing it down because it is in your inventory.

Carrying one back to a world that has never entered hardmode still works, which is how people fight the mechs out of order or help a friend get started. In the Zenith seed it stops being a summon at all and becomes a component of Ocram's Razor, the item that calls all three mechs down together.

The three mechanical summons split along a theme of darkness, light and balance. This is the balanced one, drawing souls from both sides of the hardmode divide instead of committing to either. The same idea runs through a small set of weapons built from those same souls.