The Ring, in this case, refers to The One Ring from Lord of the Rings: Tales of Middle-earth. However, we’re talking about something on a lot of other cards in the set: the phrase “the Ring Tempts you.”
The first time this happens in a game, you get an emblem in your Command Zone (which does exist even outside of Commander games). That emblem is named The Ring. This emblem has four distinct abilities printed on it, which we’ll get to in a moment. The first thing that happens each time you are tempted is that you also choose a creature you control (if you have one) to become your Ring-bearer. If a creature already is your Ring-bearer, you can choose to have it remain so, or move the designation to a different creature. The chosen creature remains your Ring-bearer until it leaves the battlefield for any reason, another player gains control of it, or you choose a different creature when you are next tempted by the Ring.
Now, those abilities on the emblem. Each time the Ring tempts you, you get access to another one, starting from the top and working your way down. The abilities are cumulative, so all the abilities that have been unlocked are active simultaneously. So, the first time you are tempted by the Ring, you get the emblem with its top ability active. Even if you control no creatures when the Ring tempts you, the emblem still gains the next ability in the order, you simply don’t get to choose a new Ring-bearer. Once it has all four abilities, the Ring can still tempt you and you can change Ring-bearers, but no new abilities will be gained and no old ones will be lost.
That’s the basics. Other cards may refer to your Ring-bearer, the emblem does give bonuses to your Ring-bearer, and some triggered abilities trigger off of the Ring tempting you. If those abilities don’t mention you choosing a Ring-bearer, they will go off even if you control no creatures, but if they do only trigger when that choice is made, you must have a creature to be your Ring-bearer for that trigger to occur. If you create a copy or another card becomes a copy of your Ring-bearer, the copy will not be a Ring-bearer itself.
The Ring is the object on which the entire story of Lord of the Rings: Tales of Middle-earth hinges, and this potentially very powerful mechanic reflects that importance. Controlling a Ring-bearer can give you quite an advantage, at the cost of that creature likely drawing any removal your opponents can throw at it. Being thematically intrinsic to the Lord of the Rings story and setting, odds are it will never show up outside of Middle-earth.
The Ring can be yours at cardkingdom.com…choose wisely!