Eldritch Moon continues the story arc on Innistrad. Innistrad had historically been stabilized by Sorin's creation of Archangel Avacyn, who was meant to keep the plane in a delicate balance by protecting the Humans from extinction. This balance has been put in jeopardy after Avacyn became inflicted with madness and a growing disgust for Humans. Ugin, Sorin and Nahiri built the original Hedron network to trap the overwhelming Eldrazi in Zendikar thousands of years ago. By now Sorin and Nahiri are estranged, and Nahiri has decided to punish Sorin for not helping her to protect Zendikar by calling Emrakul, the last remaining Eldrazi titan to Innistrad. Emrakul specializes in twisting all living beings into smaller variants of herself and her arrival on Innistrad was already foreshadowed by the madness and delirium various inhabitants of Innistrad have been struck with in the last set. Now Emrakul has arrived and we can witness the complete transformation of Innistrad's inhabitants. Liliana finally takes her oath and joins the Gatewatch, who desperately need her, as her zombies are some of the few creatures unaffected by the mind altering effects of the Eldrazi titan. Can Tamiyo help the growing team of planeswalkers to imprison Emrakul in Innistrad's silver moon?
Innistrad was already famous for its transforming creatures, but turning into a Werewolf is harmless and unimpressive compared to the reality altering effects of Emrakul. Meld cards are similar to traditional double-faced cards, but the back of these cards only shows one half of a bigger entity. You can not just transform these cards, you need to own and control the front sides of two different corresponding cards and meet the condition to meld these cards into a single monstrosity. Like double-faced cards, meld cards only have the characteristics of their front faces in every zone except the battlefield when they actually have melded together. These cards also can't be turned face down by any effects.
If seeing two of your favorite angels meld into a tentacled giant monstrosity wasn't enough to convince you that Emrakul isn't kidding, there is another gut-wrenching mechanic for you – Emerge. If you are familiar with the Alien films, you will appreciate the concept of the Eldrazi emerging out of random hosts left and right. Emerge allows you to cast some giant Eldrazi creatures for a reduced amount of mana, if you sacrifice another creature instead. You get a cost reduction up to the sacrificed creature's converted mana cost. Converted mana costs are colorless, so this doesn't affect the colored mana part of the emerge cost. Using emerge is just another method of casting a creature spell, which means it will only work during your main phase when the stack is empty, unless the spell has flash. It also counts as actually casting the spell, and will therefore put the on cast triggers on the stack as well. It works similar to a cost reduction mechanic like delve.
Innistrad has always been the embodiment of Eat or be eaten, but now conflicts are reaching a whole new level. Not long ago, you could be sure that your fellow humans would have your back against vampires or zombies, now these lines are blurring and the struggle for survival is intensifying. Escalate allows you to take desperate measures to increase the effects of your spells. It is found on modal instant or sorcery spells that offers you a bulleted list of different effects. You can choose one of the effects every time you cast such a spell, but you can also chose multiple modes if you can pay the escalate costs. These costs can differ between cards. Some Escalate cards ask you to discard cards, some to tap creatures, or pay more mana. They all have one thing in common though. You only get one effect, unless you chose to pay and escalate the situation. This is it for the completely new mechanics of the set. There are some additional mechanics though, which were already used in Shadows over Innistrad, the first set of the block.
Delirium is another take on the transformation theme. This time you do not physically flip over the card, but your cards get better when you obtain the threshold of having at least four different card types in your graveyard. Once you have reached the magnificent state of Delirium, everything appears to be possible. In Standard the seven different card types that could show up in your graveyard are creature, planeswalker, sorcery, instant, artifact, enchantment, and land. In older formats there is also a few cards with the type tribal - a discontinued card type. The important part here is that you only get to count card types in your own graveyard, and that the number of cards in your yard doesn't matter, as long as you can count at least four or more card types.
Madness is an old mechanic you might remember dominating the Odyssey block, allowing you play cards with the Madness ability although you were supposed to discard them from your hand! This weakens opposing discard effects, and also allows you to pay for your own effects requiring a discard without actually losing a card in the process. There is one minor rules change for veterans to remember though. If you discard a card with madness, you exile it instead of putting it into your graveyard, creating a trigger that will allow you to cast the spell from exile for its madness cost. If you don't cast the spell this way, it will be put into your graveyard.
Double-faced cards and meld cards break the rule that your cards have to be indistinguishable on the back while in your hand or deck. This is not a problem with opaque sleeves, which are the standard for Constructed tournament play, but in Limited events players sometimes don't use sleeves. Checklist cards are provided in Booster packs to be used as placeholders for double-faced cards and meld cards until they enter play.