Comedown.

Six friends, who've known each other from childhood, break into the tower block they lived in as kids, now deserted and condemned, to rig-up a pirate radio station, get high and party. When one of the group goes missing, her friends begin to search the dark interior of the tower and soon realize that they are not alone: a resident psychopath lurks in the shadows and is hunting them down, taking them out, one-by-one.