THE CARD
Name: Ring of Eibon
Set: Mythos Limited
Type: Artifact
Affects: Opponent's Hand
San: -1
Unique: Yes
Special Effect Box: Spend 2 Sanity points to gain a 13-second glance at one opponent's hand and choose their next legal play (cannot be a Pass) of a card from their hand to the table. Flip this card face-down after use.
CLARIFICATIONS
The crux of this card is the fact that you "choose [a] legal play... of a card from [your opponent's] hand to the table". This means you cannot force your opponent to: rotate a card, flip a card, bury a card, cast a spell, pass, or do any other action that would normally be legal. The only thing you can force them to do is play a certain card.
Although you get to select a card that must be played, you are in no way restricting the playing of other cards. For example, if you selected a Monster, your opponent may be able to Join some friends.
If a card is targetted in some way, as many Events are, particularly Phobias, you do not get to choose the target of that event. You simply specify which card an opponent must play on their next turn, not what they can do with it.
In summary: you get to choose a card that must be played on your opponent's next turn, but everything else is up to him.
QUESTIONS
Q: Can I choose to play a joined set of cards from an opponent's hand?
A: No. The Ring of Eibon only allows for "a card". However, if the card you selected was Joinable, your opponent may choose to play those other cards, if he wishes.
Q: What happens if I choose a Car, a Closed-Cockpit Monoplane, or some other card which requires for another card to immediately be played?
A: Your opponent is forced to play the first card, and then may pick the second. They must do so, and you can not choose to play that first card if there isn't a legal second card (since then it wouldn't be a "legal play").
Q: What happens if the Rounds ends before my opponent's next legal play? Or, alternatively, what happens if I use the Ring of Eibon during combat?
A: Next turn is defined as "your next natural player Turn, even if that Turn occurs in the following round". Your opponent's "next legal play", per the Ring of Eibon, happens on their next Turn. If both cases, you will have chosen your opponent's first action in the next Round. Take note that your opponent is not allowed to discard the card that you required him to play, nor is he allowed to play it before his next turn (eg, if you selected a Monster he couldn't use Create Gate to bring it into play during the Spells phase of Combat).
Q: What happens if my opponent has no legal card plays?
A: You wasted two Sanity.
Q: What happens if the card I selected becomes an illegal card play before my opponent's turn?
A: You wasted two Sanity. Your opponent gets to play whatever card he wants.
Q: What happens if I play Ring of Eibon on someone, and force him to use their Yithian Mental Contact, and he decides to Yithian the Ring of Eibon usage?
A: In practice, the Yithian Mental Contact is used, the Ring is buried, and you get the two points you spent to activate your ring back. Theoretically, you iterate through an infinite number of alternate realities until you find one where your opponent decides to Yithian the Ring, even though the Ring didn't force him to. Alternatively, the Universe may implode.
STRATEGIES
The Ring of Eibon is probably one of the most overlooked cards in Mythos. It has the potential to be devastating to your opponent's plans, though that is offset by its high cost to use. Following are a number of uses for the ring. There are many more: the Ring is one of the most versatile of all Mythos cards.
Discard a Location - Few people realize that the Ring of Eibon is a Discarder, in certain limited ways. If your opponent is walking to a Location, he is quite vulnerable. As per the most recent rules: "When walking to one Location, you might find that you wish to change to a different Location instead. Your previous destination has never been rotated right-side-up and is Discarded. It never became part of your Story. Your new Location is played crosswise, replacing your previous card." If your opponent is walking, and you choose a new Location for him to play, you implicitly force him to discard that old one.
Discard a Travel Event - The same trick can be used, to a certain degree, upon Travel Events (rules: "You may also do this if your Current Location is a Travel Event"). While your opponent is upon a Travel Event, you can pick a Location that is not legal to play from the Travel Event, but was legal from the investigator's former location. For example, if your opponent played the Mauretania, which requires a new Location card in "a different region", you could select a Location in their old region. If your opponent was upon a Tramp Steamer, which requires them to "play a new Location card featuring the Water attribute in either the same or a different region", you could select a Location without water in their old region. In both cases, your opponent would be forced to discard their Travel Event in order to play the card you selected.
Mess Up Travel Events - If you can't discard your opponent's Travel Event, you can likely mess it up. Dirigible, Seaplane, Tramp Steamer, and U-Boat are the most vulnerable to this, because they allow the play of a card in any Region. After your opponent hops on a Travel Event, ready to visit far-away lands, you select a Location in their old Region, and they're right back where they started.
Deny Attributes - If your opponent is getting ready to use Location attributes (Tome, Artifact, Gate), you can choose a new Location for them, that they'll be forced to play. This is a particularly nice (short-term) counter against Samuel Winsor or the Tiara of Opulent Fantasy.
Ward Off Passing - When an opponent begins to pass, you can use Ring of Eibon to force them to play a card. This will get you a total of one extra turn (for your two Sanity).
Set Up for Combat - Just before passes begin, you can set an opponent up for Combat with the Ring. Got Byakhee in your threat? Make them go outside. Hiding a Pack of Rat Things? Send them back inside. Have Living Dead in your Threat? Make them play a Night Event, if they have one.
Play the Wrong Card - The idea is that your opponent has some limited resource, and you force them to use it wrong. If they're at a Gate, you have them play their wimpy Skeleton rather than their Color Out of Space. If they have only one space left in their Death Tome, you have them play Orne's Black rather than that Resurrection Spell they need for their Adventure.
Play a Bad Card - Some cards will just be bad for an opponent to play at certain times. Examples include: An Unexpected Calamity, certain types of Storm (when your opponent is Travelling), Aldeberan Moves in the Sky (if your opponent has useful events out), etc. If you discover your opponent has some of these cards in their deck, you can try to get them to use them at an inopportune time. This is probably one of the most satisfying uses of the Ring of Eibon.
Cost Them Sanity - On rare occasions, you can cost your opponent Sanity with the Ring of Eibon. Force them to reuse a Gate (by selecting a legal Monster card at a reusable gate Location that costs Sanity). Have them play a costly Monster, Tome, or Artifact when they can't really afford it. Make them go somewhere that they're currently Phobic of. Cause them to leave a Sanitarium or Church before they've buried their Phobias. Choose a card other than a Location for them to play when someone else has Townsfolk Rioted them.
Intelligence Gathering - I've primarily talked about the use of the Ring to force your opponent to do stuff. It's other power, the fact that it allows you to see your opponent's hand, should not be forgotten. You can see: what Monsters are going to end up in your opponent's Threat, what Adventures they're working on, how close they are, and lots more. Ring of Eibon works very well with Amnesia. After you verify that your opponent is actually holding crucial cards for their Adventure, then you can nail them with your Amnesia card.
COUNTER STRATEGIES
Situations where you can counter the Ring are rare. Most of the time, you'll be stuck.
Typical Artifact Strategies - Steal the Ring with Mi-go or Thieves in the Attic. Decrease your opponent's Sanity level so that they can't afford to use the Ring.
Yithian It! - If you're hit with the Ring of Eibon, you can't Yithian it, since your next card play has already been selected, and then it's too late. However, if an opponent is using the Ring to very good effect, and hits someone else with it, it might be worth you, an unaffected third party, using a Yithian. The Ring is flipped when it's used, and that's an orientation change, so when it's directed at someone else, you can toast it.
Indirectly Make Your Card Play Illegal - This only works if you're hit with the Ring, and then the round ends before your next turn. You can't discard the required card, but you can do some sneaky, indirect thing during the Artifact/Tome phase of combat if you really don't want to play it. The most obvious thing you can do is change your Location with the Create Timewarp or Fly spells. Simply go somewhere where you can't play the Ally, Tome, Artifact, or Monster that was selected. If your opponent selected a Tome, you might get rid of the Allies who knows its language with Body Warping of Gorgoroth, Call Power of Nyambe, Devolution, Consume Likeness, or Dread Curse of Azathoth. If your opponent selected a weapon, you might kill every single one of your Allies in similar manners (though that's likely overkill).