Zombie Elf

They exist only inside the Frost Moon, which you start yourself with a Naughty Present at night once Plantera is dead. Nothing brings them out on a normal night, and dawn ends them along with the rest of the event no matter which wave you had reached.

This is the event's plain ground walker: no projectiles, no jumps, it closes and hits. A platform a few tiles off the ground blanks them completely, which is a large part of why every Frost Moon arena is built the same way. The threat is arithmetic rather than skill. Contact damage is heavy for filler, and in the late waves they arrive faster than most builds clear them, so getting boxed in on the floor ends runs.

Every kill nudges the wave meter along, and the harder difficulties credit more progress per body, so a Master world climbs into the dangerous waves earlier than a classic one. While Everscream, Santa-NK1 or the Ice Queen is on screen, though, those are what decide whether the meter keeps moving, and the elves are cleanup between them. Their banner takes well beyond a normal kill count, with every elf variant feeding the same total.