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. The Zendikar storyline continues in Shadows over Innistrad, as the vampire Sorin is sent by Spirit Dragon Ugin to look for Nahiri. These three planeswalkers built the original Hedron network to trap the overwhelming Eldrazi in Zendikar thousands of years ago. By now Sorin and Nahiri are estranged, and it looks like Nahiri might even have fallen under the influence of Emrakul, the last remaining Eldrazi titan. Emrakul specializes in twisting all living beings, and her arrival on Innistrad would explain the madness and delirium various other inhabitants of Innistrad have recently been struck with. The name of the next set in the block and a series of clues hidden on various cards in SOI also hint towards Emrakul...
Innistrad is a dangerous place, which puts a lot of evolutionary pressure on its inhabitants. Unsurprisingly, the remaining creatures of this plane have developed a lot strange abilities to have an edge in their daily struggle for survival. Between Humans turning into Werewolves, Vampires transforming into Insidious Mist, and Frozen Krakens awakening from icy hibernation, there are a lot of transformational processes happening. Once again we will see these processes visualized by double-faced cards. These cards don't have a traditional Magic card back, instead they have a front face and a back face, and a card specific rule how you can transform the card, which means turning it into whatever is on the other side of the card. You may be used to Double-faced cards by now thanks to the flip-walkers from Magic Origins, but there are some minor rules changes to remember. The converted mana cost of a double-faced card is always the mana cost of the front face now, which makes more sense than becoming a 0-drop for technical reasons. If a double-faced card has an ability that transforms it, that permanent transforms only if it hasn't yet since that ability was put on the stack. Imagine you want to transform your card, but your opponent responds with a conditional removal that would not kill your creature once transformed. The new rule allows you to respond to your opponents removal with another transformation attempt, without causing the first attempt that is still on the stack to transform your creature back to the front side again when it will finally resolve.
Double-faced cards break the rule that your cards have to be undistinguishable 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 until they enter play.
If you thought Innistrad was wild the last time, you should better prepare for a whole new level of madness now. 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.
This creature can't be blocked by creatures with greater power. While most of the creatures on Innistrad are strong and durable, there are other ways to be successful in this dangerous world filled with maddened monsters. Sometimes it is better to just sneak past the more unpleasant residents of Innistrad, and the Skulk keyword can help your creatures to do just that.
Delirium is another take on the transformational 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, 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.
Since our last visit to Innistrad, various strange things have happened. There seem to be great powers at work distorting not only the physical world, but also the minds of Innistrad's residents. Even the Archangel Avacyn herself, who was keeping the plane in a balance, has descended into madness. Who is behind all these changes on Innistrad? We probably won't know for sure before Eldritch Moon, the second set of the Shadows over Innistrad block will be released, but to start solving these mysteries, you'll need to investigate and follow the clues. Which brings us to our last new mechanic - Investigate. Some SOI cards tell you to investigate, which means Put a colourless Clue artifact token onto the battlefield. Such a Clue always has the text 2, Sacrifice this artifact: Draw a card..