Red light, a y that has an explosion attack that causes paralysis. This is a reference to red lights in real life making you stop.

(There could also be green lights whose explosion gives a temporary speed boost, but since this isn’t harmful to the target it’s a sillier idea.)

Make shopkeepers more susceptible to being paralyzed or put to sleep than they currently are, so it is easier to directly but non-lethally steal from them.

Paralysis should halve your speed and give you a 30% chance of not moving on a given attempt, rather than just being “sleep but harder to resist and you can’t wake up”.

Paralysis only freezes you in place on a square; it doesn’t prevent you from attacking and taking other non-movement actions.

Add a spell of paralysis, which basically copies the same monster spell. Melee range, better than sleep.

Applying a blessed whistle (either type) will unparalyze pets in range.

Ghosts lose their physical touch attack in favor of a reusable attack that scare-paralyzes you. Possibly, if this is unflavorful due to it just being the same ghost, change their AI to make them turn invisible after a certain amount of time being visible and not next to the player. Then they path towards you, and if you either stumble into them or they are next to you and get a move, they turn visible, paralyzing you. Since the actual action of them appearing out of nowhere would be what frightens you, see invisible would negate the effect.

Skeletons, or any monster with the MS_BONES sound, rattle their bones periodically, which frighten the player into helplessness for a few turns.

New monster: green cube, which combines the abilities of a green slime and gelatinous cube: both sliming and paralysis.