Not an item, but a barbarian's relentless rage

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