There are a bunch of haunted forest modes---haunted scoring for shots, targets, etc. (They're listed on the sign.) One is lit at the VUK after seven pop bumper hits (by default), spelling "HAUNTED" in purple on the arrow that points at the pops.
After you get all the modes on the sign done, the next one is haunted multiball.
Think Otto modes and Springfield Mystery Spot from Simpsons PP.