Here is another installment of exTRAPaganza, our Thursday over-the-top trap series.
I am watching the Twilight Zone
Pick one PC. Out of the corner of his eye he sees movement and discovers something that looks like a monster that is a few CR higher than the APL. However, no one else can see him. So, no matter who he tells, it’s not there, until, at some point, when they are in a big fight, the monster appears making guerrilla attacks and trying to not be seen by the other PCs. A ranged attacker works best.