Robotic Skull

The pet that comes out of this is a shrunken Skeletron Prime that trails behind you with its arms flailing. The buff description makes it sound like the little machine fights on your behalf. It does not. Nothing it touches takes damage, so treat it as a trophy you can carry rather than a helper.

It only comes off Prime in Master Mode, and not on every kill, so expect a run of repeat fights. Mechanical Skulls are craftable, so the sensible approach is to build a small arena, keep a stack of summons on hand, and reset the fight until it drops.