Etherian Javelin Thrower

It exists only inside the Old One's Army. Nothing about your world will produce one otherwise, and it never wanders in by itself. Which waves it joins depends on the tier: the opening tier holds it back until the event is already underway, while both Hardmode tiers send it in from the first wave and again near the end. It arrives in smaller numbers than the Etherian Goblins and bombers around it, so its banner takes several times the usual grind to earn.

The javelin is a slow arcing throw on a loop rather than a stream, so one thrower left alone is a steady drip of damage instead of a burst. The danger is what it aims at. It will happily wind up on the Eternia Crystal from well outside the range you are personally defending, which means any that slip past your line keep chipping away while you are busy at the other side of the arena. Kill them the moment they get behind your traps and sentries. Up close they are heavy and slow to turn, so knockback or a wide swing clears them.