One Ring to Confuse Them All
One Ring to draw them all the cards, One Ring to protect them,
One Ring to confuse them all and in the judge call bind them,
In the Land of Modern where the Death's Shadows lie.
The One Ring is a complicated card. Judges and players alike frequently make incorrect assumptions about how it works, which, given its prevalence, is not ideal. This is a brief but hopefully-comprehensive guide to rules and policy interactions with The One Ring; if you notice anything missing, please let me know so I can add it.
Let's go through the abilities one at a time:
"Indestructible"
Ok, this one's pretty straightforward. The One Ring cannot be destroyed. Anything that uses the word "destroy", along with lethal damage if it somehow turns into a creature, will not destroy it. Anything else that tries to put it into the graveyard will still succeed. Moving on.
"When the One Ring enters the battlefield, if you cast it, you gain protection from everything until your next turn."
This is not a "cast trigger" like on Ulamog, the Ceaseless Hunger. This triggers after The One Ring has resolved and entered the battlefield, but only if it got there by virtue of you casting it as a spell.
"Protection from everything" means that you can't be the target of any spells or abilities, nor dealt damage or enchanted. It doesn't do anything else! It doesn't prevent you from being attacked, or your spells from being countered, or you from being affected by an "each opponent" effect. (Notably, you're only the target of a spell if it actually uses the word "target".
You gain protection "until your next turn", which specifically means the beginning of your next turn. Once your next turn starts, you no longer have protection.
Sanctioned tournaments, such as a Regional Championship Qualifier, have special rules for triggered abilities. (Any ability that starts with "when", "whenever", "at the beginning", or "at end of combat".) Players are not allowed to intentionally miss their triggers, but if they do it by accident, their opponent isn't required to remind them. And if it's discovered later (within a turn), it's that opponent who gets to choose whether the trigger happens.
Here's how it works: Triggers are assumed to be remembered until their controller takes some physical action or makes a verbal statement that indicates it was missed. In other words, if the game state is ambiguous and there are multiple possible interpretations, we assume the trigger was not missed. But if they allow the game to enter a state that could not have been legal if the trigger had been remembered, then it was missed.
This is the case for all triggers; The One Ring is not special in this regard. For example, if Alice controls Monastery Swiftspear and casts Lava Spike without saying anything about the trigger, then later announces 2 damage, that's fine; nothing she did ever indicated that the trigger had been missed, so it wasn't. But if she announces 1 damage, now she's clearly indicated that she's missed the trigger, because if it had resolved, Monastery Swiftspear would have had 2 power, not 1. Similarly if she allows the opponent to target Monastery Swiftspear with Bala Ged Scorpion, she's missed it. If she wants to ensure she doesn't forget about it later, she can simply announce it when it first triggers, at which point it can no longer be missed.
Another example: Alice casts Bloodbraid Elf, not mentioning cascade. Nathan says "it resolves", and Alice starts performing cascade. Here too, Alice has not done anything to indicate the trigger has been missed, so nothing has gone wrong here. (Nathan claiming that Bloodbraid Elf resolves doesn't make it true; Nathan doesn't get to just choose for Alice's trigger to disappear. His action was just to pass priority, and the trigger is the next thing that should resolve.) But if she had said "ok, it's on the battlefield" or tapped it to attack with it, then she's missed the trigger.
The One Ring works exactly the same. If the player announces "I have protection" or similar, it's not missed. If they don't mention the trigger and bring it up when it's first relevant, it's not missed. If they fail to bring it up when it's relevant, and allow themselves to be targeted or dealt damage or enchanted, it is missed.
"At the beginning of your upkeep, you lose 1 life for each burden counter on The One Ring."
This is another triggered ability, and it can be missed just like the first. However, since this trigger is bad for the controller, missing it comes with a penalty of a Warning in any Competitive REL tournament, and the opponent would want to put it onto the stack. In addition, it becomes relevant immediately upon resolving (since losing life is a visible change to the game state), so there aren't usually many arguments about whether it's been missed.
(If there are no burden counters on it, the player doesn't have to mention it, since it would do nothing.)
One question that does come up is: What happens if The One Ring leaves the battlefield in response to the trigger? When the trigger resolves, it tries to make the player lose life equal to the number of burden counters on The One Ring. But The One Ring doesn't exist anymore, so the game uses its "last known information"; it checks how many counters were on it the last time it was on the battlefield.
This is, again, nothing special to The One Ring. When Hangarback Walker dies, its trigger doesn't resolve until after it's dead, so the game looks at how many counters were on it the last time it existed on the battlefield. (The card wouldn't work at all otherwise.) This is a very broadly-applicable rule, which applies to any card with an activated or triggered ability that references information about itself when it resolves.
"{T}: Put a burden counter on The One Ring, then draw a card for each burden counter on The One Ring."
The main thing to remember with this ability is that the burden counter is put on The One Ring as the ability resolves, not when it's first activated. If The One Ring is no longer on the battlefield when it resolves, a counter can't be placed on it, and then the game uses its last known information to determine how many cards the player will draw, just like the previous ability.
Misc.
Everything you know is a lie; there are two rings.
Luckily the Infraction Procedure Guide has us covered:
Truncated names of storyline characters on decklists (legendary permanents and planeswalkers) are acceptable as long as they are the only representation of that character in the format and are treated as referring to that card, even if other cards begin with the same name.
An inanimate piece of metal is not really a "character", but the IPG clarifies that it refers to any legendary permanent, so this is fine. Anything vaguely similar to "The One Ring" will be interpreted as referring to it, even if the exact text is a little different.
Examples
It's always good to practice knowledge to make sure one understood it correctly, and people remember things better if they interact with them multiple times and turn them over actively in their heads, so here are some example questions. For each of them, I'd encourage you to not just click through to the answer, and instead try to figure it out yourself first.
Question #1
Alice controls The One Ring with 3 burden counters on it, and casts another The One Ring. What happens?
Due to the legend rule, she must choose one of them to put into the graveyard. The legend rule doesn't destroy the permanent, so indestructible doesn't prevent this.
Question #2
Alice controls The One Ring with no burden counters on it. She activates it, and then in response she untaps it with Minamo, School at Water's Edge and activates it again. How many cards does she draw?
Three. When the first instance of the ability resolves, it puts a counter on and she draws a card. Then when the next instance of the ability resolves, she puts another counter on and draws two cards.
Responding to the ability with itself would have helped if the counter were put on as a cost, since then both counters would be on it before either resolves, and she'd draw four cards. But since the counter isn't added until it resolves, this doesn't do anything different from activating them one at a time.
Question #3
Alice casts The One Ring, and Nathan gains control of it with Commandeer. Does either player gain protection? If so, who?
Neither gains protection. The One Ring enters the battlefield under Nathan's control, but Nathan didn't cast it, so it doesn't trigger.
Question #4
Nathan has protection from everything. Can Alice attack him with Questing Beast? If so, does he take damage?
Yes and yes. Protection doesn't prevent attacking. It does prevent damage, but Questing Beast's ability overrides that and says the damage can't be prevented, so Nathan takes 4.
Question #5
Alice casts The One Ring, not mentioning anything about its trigger. On Nathan's turn, he casts Lava Spike targeting Alice, and Alice says he can't do that because she has protection. What happens?
Alice has demonstrated awareness of the trigger at the first time it mattered to the game state, so it's not missed. Nathan isn't required to remind Alice about it, which means he's allowed to play as though it it was missed and see if Alice notices, so he gets no penalty for this. The Lava Spike is rewound and he doesn't have to cast it.
Question #6
Alice hits Nathan with Ragavan, exiling The One Ring. If she casts it, will she gain protection from everything?
Yes. When it enters the battlefield, Alice had cast it, so it triggers.
Question #7
Alice casts The One Ring, gaining protection. On Nathan's turn, can he kill one of Alice's creatures with Terminate? Can he cast Exsanguinate?
Yes to both. Only Alice has protection, not her permanents. And Exsanguinate doesn't target nor damage Alice, so can be cast and resolve as normal, making her lose life. (Only effects that actually use the word "damage" are doing damage.)
Question #8
Alice casts The One Ring, saying "I gain protection". On Nathan's turn, he attacks and says "you take 5", hoping she forgets about it, and she calls a judge. What happens?
Alice had clearly demonstrated on her turn that the trigger resolved, by stating that explicitly. Nathan has therefore done something illegal, and in a Competitive REL event will receive a Warning for a Game Rule Violation. (Or be disqualified for Cheating, if he knew this was illegal.)
The rule is "players are not required to remind their opponents of their potentially-missed triggers". Once Alice has made it clear that the trigger hasn't been missed, Nathan doesn't get to keep trying to sneak something through.
Question #9
Alice controls The One Ring with 2 burden counters on it, and activates its ability. In response, Nathan exiles it with Cast Into The Fire. How many cards does Alice draw?
Two. When the ability resolves, it can't put a burden counter on The One Ring, but it can check The One Ring's last known information and see that it had 2 counters on it right before it got exiled, so Alice draws that many cards.
Question #10
Alice casts Phyrexian Metamorph, copying Nathan's The One Ring. Does she gain protection?
Yes. Whenever a card refers to itself by name, it just means "this object". Alice did cast that object, and it doesn't matter if it happened to be named "Phyrexian Metamorph" at the time.
Question #11
Alice casts The One Ring, not mentioning the trigger. At the end of her turn, Nathan looks at the top card of her library with Mishra's Bauble. On his turn he attacks, and Alice says that she has protection. A judge is called, and Alice explains that she thought that Mishra's Bauble said "target library" and so her protection wouldn't prevent it.
By allowing Nathan to target her with Mishra's Bauble, she's demonstrated that she's missed the trigger. Whether or not she knew about the trigger at the time is irrelevant; it's "missed trigger", not "forgotten trigger". We can't read people's minds, so we don't take into account whether they knew about the trigger inside their heads; all that matters is the visible actions they perform and words they speak. Alice doesn't have protection, and Nathan will be able to deal combat damage to her on his turn.
Question #12
Alice casts The One Ring, gaining protection. On Nathan's Turn, he casts Nexus of Fate. Will Alice have protection on Nathan's extra turn?
Yes. She has protection until her next turn begins, regardless of how many other turns there are in between.
Question #13
Alice casts The One Ring, saying nothing about its trigger. At the end of her turn, Nathan targets her with Lightning Bolt, and a spectator says "doesn't she have protection?" What happens?
If the spectator does this right away, the spectator has committed Outside Assistance by reminding Alice of her trigger. In a Competitive REL event, the spectator will receive a Match Loss in the next match they play. There's no way for the judge to know whether Alice would have missed it on her own, so we can't undo the information that's been gained, and Nathan won't be allowed to target her with Lightning Bolt.
If Alice had already indicated that the Lightning Bolt was ok, such as by saying "I take 3" or starting to edit her life pad, and then the spectator spoke up at that point, she's already missed the trigger. The spectator's interference is not going to go back and undo that, so she doesn't have protection and Nathan will be able to deal damage to her on his turn.
Question #14
Alice is at 6 life and controls The One Ring with 5 counters on it. In her upkeep, with the trigger on the stack, she activates it to draw cards. She responds to that ability by turning The Mycosynth Gardens into a copy of it and choosing to put the original into her graveyard. When the abilities resolve, how many cards does she draw and how much life does she lose?
She draws 5 cards and loses 5 life. Taking things in order: First the activated ability resolves. She's unable to put a burden counter on The One Ring, so that instruction is ignored. Then she's instructed to draw cards equal to the number of burden counters on it, so the game looks back at how many burden counters were on on it and sees that it was 5. Then the same thing happens when the trigger resolves, making her lose 5 life.
(Any time a card refers to itself by name, it means "this object", not "any object with this name". The existing abilities on the stack refer to the Ring that was the source of those abilities, they don't switch to referring to the new One Ring.)
Question #15
Alice controls Elesh Norn, Mother of Machines and casts The One Ring. Does she gain protection for two turns?
No. Elesh causes it to trigger twice, so there are two triggers that give Alice protection until her next turn. Nothing says that they'd extend any longer than that, so they don't. Having two instances of protection doesn't do anything different from having one instance, so there's no relevant difference to just having one trigger.
If you have any further questions, feel free to ask in the comments. For urgent situations, you can also ask in the judge chatroom, which is open to everyone and generally pretty reliable.