A “repulsion” intrinsic. This prevents monsters that fail a resistance check from moving next to you (specifically, where they believe is next to you) on their turn by making their AI consider it an invalid position. However, if you are in melee range, it does not make them step away from you or refuse to fight you, nor does it prevent them from making ranged attacks.

This intrinsic could come from some new artifact or spell, but two possible sources for it that already exist are reading the scroll of scare monster and casting the spell of cause fear.

Any monster that is somehow forced to stand on a scroll of scare monster while being affected by it is immediately scared to death.

Scrolls of scare monster that have the “already picked up once” flag set are given some descriptor that can clue players into which of two stacks has been picked up or has not. However, this is only shown when scare monster is identified, so it can’t be used to trivially identify the scroll without letting one turn to dust.

If there is a scroll of scare monster in a container, monsters will still open the container, but will not take any items out of it.

Make a radical change to the scroll of scare monster, making it no longer scare monsters on the ground; instead, boost its reading effect to a very powerful scaring effect, more so than can be accomplished with a horn.

If a player fear system is implemented: if the player is adjacent to a creature standing on a scroll of scare monster, they will become scared of that creature. Intelligent creatures will deploy a scroll of scare monster to take advantage of this.

Revamp the scroll of scare monster: rather than being good indefinitely as long as it’s not picked up, it will increment an internal counter whenever it scares a monster, by an amount equal to that monster’s HD or difficulty. The scroll disintegrates on the floor after this “total difficulty of monsters scared” counter exceeds some value.

If scare monster is not formally known, change “The scroll turns to dust as you pick it up” to use the called name or label, e.g. “The scroll labeled NR 9 turns to dust as you pick it up”.

Mindless undead are immune to every form of fear effect, including scare monster.

Note: EvilHack takes this a step further and makes every mindless monster immune to fear, not just undead. This may be more apt, depending on your point of view.

Writing “Elbereth” in the prompt for writing a scroll should alias to “scare monster”.

The scroll of scare monster, on the ground, will do a check versus monster magic resistance before it actually scares a monster.

The list of monsters that ignore Elbereth should be increased, but the list of monsters that ignore scare monster should remain the same or decrease (3.6.0 Quest nemeses excepted). Perhaps scare monster should be renamed to “scare creature”, as it scares humans and elves, which are not really monsters.