Need an idea for an item that's irresistible to use, but has consequences

Basically what the title says. I want to come up with an item that's given to the player by someone that can't be trusted. The item has to do something that just seems too good to be true, and they'd want to use it right away, but then the item does something in return that has lasting consequences.

Any ideas?

So what you basically mean is that the item has both good and bad effects on the story/character, right? If so, here's some suggestions -
1.Get the item required to catch a certain legendary pokemon, and when you use it, the pokemon will cause chaos and destroy PCs (or maybe not) and you'll no longer be able to heal your pokes. (for a limited time of course)
2.Get a Fire/Water/Grass/Any other type ID card which boosts all stats of the pokemon of the corresponding type, but you won't be able to use pokemon of any type other than the card's.
3.Get a potion that never ends (or gets refilled after a certain amount of time), at the cost of never being able to buy any other healing item again.

Well, these items were of course a bit too much, EXTREMELY TOO MUCH. If I get a rough idea of the storyline, or maybe the character, then I could share some other ideas too.

A couple more simple ideas (though maybe not so simple to actually execute)
a. A complete full heal/PP restore of one Pokemon in your party, but you can't use any medicinal type of item on that Pokemon until resting at the PC
b. An item that causes your Pokemon to gain 3-5 levels, but disables the ability for them to get xp until you do X or Y.
c. Max out a select Pokemon's EVs. However, their IVs are all set to 0.

Something like that maybe? Interesting question

Here's an idea
Ancient onyx
An item to be held by an onix. Doubles defence and special defence or multiplies attack and special attack by 1.75 (whichever one is higher) at the cost of your speed being halved.

It really depends on how much freedom you have to implement the item effects. These are a few suggestions:
1. An item that gives PoKérus to a Pokemon but won't allow the Pokemon to level up until the next gym is beaten
2. An item that multiplies the experience gained by 4 but makes the Pokemon unable to evolve

A key item that you can activate that turns into another item (from a inactive state to a active one or breaking open a box for a item inside for example), which gives you access to a something exclusive like a small area that has a bunch of goodies in it (that you otherwise can access much later). But then later the game will check if you still have the original item and change what it does accordingly.