So I was thinking maybe some normal appearing horses could spawn near lakes and rivers? But not in groups, just a single horse. This would raise the players suspiscion, maybe they could have red eyes. If you mount the horse it'll transform into a kelpie and give you the slowness and blind debuffs and start into the water, you can kill the kelpie while on its back but it will drag you into the nearest body of water and sink to the bottom to drown you. However im not sure how hard this would be to program.