Mechanical Eye

Night only, but the biome does not matter, so you get a free choice of arena. Build somewhere long and flat with plenty of room above you, because the pair you are summoning splits up and attacks from two directions the moment the fight starts. Trying this in a cramped tunnel is how the fight ends early.

You do not have to craft one at all. Before you have beaten the Twins, any Hardmode enemy that drops coins can hand you one instead, so a long farming session often produces one on its own. Crafting is just the faster route when you want the fight now.

• The three mechanical summons split along a light, dark and balance theme, and this one is the light entry. The same three ideas repeat in a set of weapons made from the souls those bosses drop.
• It is the only mechanical summon whose sprite has no visible winding key.
• Carrying one into a world that has not entered Hardmode still works, and so does taking it to a friend's fresh world.
• In the Zenith seed it stops being consumable and becomes a component for the summon that calls all three mechanical bosses at once.