This is for achieving 4 of the Hell tower achievements. You will be able to check them by hitting ESC then the spiky ball. Click the drop down menu and select Halloween Pack. Helltower achievements will all have Helltower: in the title.
EDIT: Uncle Dick has brought to my attention that this achievement may be buggy. He had to get 5 for it to unlock. I went back and confirmed my completion times and they support that claim. It didnt pop until I had 5. So if it doesnt pop, just get another.
If someone manages to get in with 4. Please post in the comments the 4 achievements that triggered it as one might not be registering and it would be helpful to future readers find out which.