Rapid-fire teleporting, then a kick.
Null Was Here teleports around the player at a punishing rate, roughly once every 4 game ticks, while cycling through unsettling title messages: "CORRUPTED," "NULLNULLNULL," "Behind you," and others.
At the end of the sequence it kicks the player from the world with "You are not whitelisted", once in safe mode, three times in a row in unsafe mode.