I often make up artifact-level magic items for my campaign. In general, they are more of a McGuffin of limited utility rather than The Ring of Power. I really suggest using the artifacts in the DMG as a guide rather than items to be selected.
The Crown of Saunders is one such item. The Kingdom of Saunders is a tiny place. Its main claim to fame is that the wisdom of the kings has allowed it to remain stable for centuries. This is the power of the Crown, the actual crown worn by the king.
Saunders' main problem is that its princes usually fall prey to the men around the king, the type of men who always gravitate to power. Every prince has always been remembered as a horrible little prick, who, upon ascending the throne and donning the crown, suddenly had a change of heart and became wise and kind.
This is the Crown's doing. It has the following magic:
Reverse Alignment (once in a lifetime),
Immunity to Charm,
Allows the spell ability of a 9th-level Magic-User,
Confers on the owner the ability to sing, swim, ride horses, and dance exceptionally well.
Confers the ability to speak and understand many languages.
Confers a +1 to Charisma, up to 19.
The first 3 powers require the wearing of the crown. If someone other than the king wears the crown, they will experience the alignment reversal, but not gain any of the other abilities. Those are reserved for the king.
The DM should make up a list of spells for the crown as if a Magic-User were getting ready for travel, not combat. These can be cast once per day, but only if the crown is worn. These extra spells require no study or a spell book. Should the king actually be a Magic-User, that person can cast their own spells or one of the ones the crown "knows", but they cannot exceed the casting limits of a 9th-level caster. They don't get to cast extra spells. The Magic-User is required to study if they wish to cast a spell from their own spell book as per normal.
The next three powers are conferred to the proper owner of the crown, not someone wearing it or holding it. It is impractical to wear the crown all the time, especially when dancing or horseback riding. The Kings of Saunders can perform all of these tasks ridiculously well, but I will leave the actual bonus up to you. I will note that drownings and riding accidents never happen to the Kings of Saunders; it's like they are Olympic-level athletes. Hubris is still possible, but unlikely.
DMs should select up to 3 languages for the Crown to confer to the King, based on your campaign setting. In my setting, these are "evil" languages that a King typically wouldn't study due to the stigma.
Several wily kings have historically used the alignment-reversing power of the crown to "pardon" a politically motivated criminal by placing the crown on their head. This will force the alignment change, turning an evil or chaotic person into a lawful or good citizen. It greatly reduces the likelihood of reprisals and problems for the king. The utility of this is dubious, as the reverse of neutral is still neutral, and so is reversing the alignment of someone who is acting out of logic or belief. On many occasions, this trick of "pardoning" has actually caused more problems for the king than it solves.
So, here you have it. A very neat and powerful item that could, but doesn't have to be game-breaking. I will have a second historical post as to how this Crown was created. It is unusual because the Crown's gambit of powers are also unusual.