The upgraded form of this is a reverse time on the entire enemy field.
...That's the most OP card idea I've ever encountered. xD
My opinion: Stick with the theme, but make a completely different card.
Maybe a creature affected with Fear can be too scared to attack and susceptible to death. Compare to freeze+shockwave. A frozen creature is delayed for 3 turns, and if shockwave is used on a frozen creature, it is instant killed. Maybe Killing Fear/Horror delays a target creature for 1 turn, during which they are too afraid to attack. Any CC used on an afraid creature kills it instantly. Alternatively, maybe only a drain life played on an afraid creature can instant kill it.
I feel like this provides that CC that you're looking for while avoiding stepping into
territory. It does make for, well, the same effect as freeze+shockwave though...But it is just an idea, after all. I'm sure it could be balanced or tweaked to make it useful in it's own right without over shadowing the aforementioned combo.