Poltergeist

It appears only during the Pumpkin Moon, and not from the start of it. The event has to grind up to its fourth wave before poltergeists join the roster, which puts them well past Plantera in a normal run. Nothing summons one on its own and they never spawn outside the event.

It used to behave like a Wraith and needed ground underneath it to drift along. That is no longer true: it goes anywhere in open air, which killed the old habit of building a platform high enough that the walking half of the event could not follow. In exchange it carries far less health than it once did, so more of them reach you and each dies faster.

Each one is worth a decent slice of wave progress, so killing them is how the night advances rather than something you do to survive it. They arrive in crowds, and a build that cannot clear a crowd stalls out at this point. The banner also asks for double the usual kills, which is the balance for how many the event throws at you.

In folklore a poltergeist is the ghost that hurls the furniture around. This one is drawn hauling a book, a table and a chair, and it cannot throw, drop or move any of them.