What is the best way for an evil NPC to avoid the effects of a Talisman of Pure Good?

Question

I'm a first time DM and perhaps foolishly gave one of my PCs (whose backstory is that his family was killed by a demon) a talisman of pure good after completing a quest. Now I am regretting it, because if I put the party up against an evil creature, they can pretty easily get rid of it by just using the talisman.

The wording of the talisman of pure good says:

you can use an action to expend 1 charge from it and choose one creature you can see on the ground within 120 feet of you. If the target is of evil alignment, a flaming fissure opens under it. The target must succeed on a DC 20 Dexterity saving throw or fall into the fissure and be destroyed, leaving no remains. The fissure then closes, leaving no trace of its existence.

A couple questions on that:

1. Can a flying evil creature simply fly over the fissure, and thus avoid the saving throw and the threat?
   Nothing in the wording says the creature is sucked or pushed into the fissure, which seems to imply that a creature not on the ground could easily avoid it.
2. Does a demon/devil on the material plane get completely destroyed, or simply sent back to the Abyss/Nine Hells?
3. Are there any other strategies I'm not thinking of to get around my earlier foolishness in giving my PCs such a powerful artifact?

Answer 1

OK, you gave them something pretty powerful and now regret - been there.

Options to stop them using include:

Let them use it

It only has 7 charges and is then destroyed so throw some evil creatures at them, presumably powerful ones, so they are tempted to use the charges. Throwing a large group of lesser evil enemies at them might make them opt to use it to simply reduce the number of enemies they face. If they are clever though, they will hold back on the last charge as a hail Mary should they ever need it.

Steal it

PCs hate having their gear stolen, and it does cross the line of removing player agency if you do it wrong, but items can always be taken from players. A lucky thief or someone who is after the talisman in particular can attempt to take it. Note that if you do this the PCs will move heaven and hell to get it back. Or, at least, that has been my experience any way.

Legendary Resistances

When this creature fails a save, it can choose to pass it instead.

This is a great way to make a boss tougher and works really well for your particular situation.

To answer your flying question, the talisman says:

[...] expend 1 charge from it and choose one creature you can see on the ground [...]

So the target must be on the ground, standing, lying, sitting, etc. Creating a fissure under a flying creature does nothing. A creature that can fly but currently isn't (e.g. is on the ground) is not safe either.

Lastly, is the demon destroyed utterly or banished? The DM may have a say in this but generally a demon is only banished from the plane it was on upon its death. Although the description of the talisman says the demon is destroyed I believe this wording is simply to make it clear that there are no remains for the PCs to claim or loot - it did fall down a large hole that closed up again.