This ghost looks like a regular human until you get a closer look at it and stumble upon a horrifying revelation - it has no face, and it's actually a spirit! Thus, it forces the creature to be horrified and incapacitated by what it sees. It's a somewhat magical effect, I would imagine, because it can repeatedly do this to a creature.
The conditions I set of the effect activating whenever it taps and then lasting until it untaps is a flavorful one. The ghost here hides its face under a cloak (untapped). Then, whenever it reveals its face, it horrifies whoever it is showing its face to. The cloak stays open while its tapped. The moment it untaps is when it puts the cloak back over its face.
This effect is indeed similar to detain and is a color bleed for black for the sake of flavor. Fear functions similarly with evasion, but not letting a creature be able to attack feels out of character for black. Is this too much of a violation of the color pie?