In my current 5e game, I have some items that have soft limits on how often you can use them, in that each time you use them you make a con save or gain a level of exhaustion, and the dc increases as you use them more, and the dc resets when you finish a long rest without any exhaustion.
they’re epic level, they can probably handle it.
on the other hand, if they fight Szass Tam or Sul Khatesh…
Since like, the 70s
Perhaps the party has played Baldur’s Gate 2
Iirc, it’s a children’s book about an elephant named Horton discovering a civilization of tiny creatures living on a planet the size of a speck of dust and protecting it from other animals.
There’s one that lets a player make up to 6 extra attacks when they take the attack action(declaring the number of extra attacks first), but after each attack they make the con save, and the dc starts at 10 and increases by 2 each time. I also had another one that allows for casting of spells up to 6th level without spending spell slots, with a number of saves equal to the level of the spell