Goblin Warlock

She only appears inside a goblin invasion, and only once the world has entered Hardmode. She is rare within the event, so most waves will not have one, and there is no item that calls her out directly. If you want more of them, kill only warlocks and leave the rest of the wave alive: each one you drop moves the invasion counter by a fixed slice, and a single event has room for roughly a dozen of her before it runs out.

She opens like any other ground goblin, walking at you. Once she is close enough the fight changes shape. She conjures a shadowflame apparition over her head, climbs straight up until she is above you, dashes across to wherever you are standing, then drifts back down in a slow squared off loop. Blocks stop none of it. While she hovers she throws shadow projectiles at a steady rate, and those can be knocked out of the air by any weapon or shot you put in front of them. The apparitions are the actual damage. She is a delivery system for them, and every second she spends stationary is another one in the air.

Keep her moving and she never gets to summon anything. That is the whole fight. Build a long platform well above the ground, run along it with speed boots or a mount, and shoot backwards; she spends the entire time chasing and never plants herself. Knockback does the same job in a smaller space, since enough of it stops her climb before it starts, so a fast ranged weapon with push is worth swapping to the moment she appears. The other option is to knock her down through the platform, because she cannot get back onto it while you are watching her.

The shadowflame weapons are the reason to fight her at all. They inflict the shadowflame debuff themselves, and at the point in Hardmode where a goblin invasion is realistic they are a real step up rather than a sidegrade. She never uses a single one of them against you, including the hex doll drawn into her own sprite.

She is the only goblin exclusive to Hardmode and the only one with her own hurt, death and idle sounds. Slow the death sound down by about half and it turns into a shout for help.