Shadow Slice — ruling 1

The spell with cipher is encoded on the creature as part of that spell’s resolution, just after the spell’s other effects. That card goes directly from the stack to exile. It never goes to the graveyard.

Shadow Slice — ruling 2

You choose the creature as the spell resolves. The cipher ability doesn’t target that creature, although the spell with cipher may target that creature (or a different creature) because of its other abilities.

Shadow Slice — ruling 3

If the spell with cipher doesn’t resolve, none of its effects will happen, including cipher. The card will go to its owner’s graveyard and won’t be encoded on a creature.

Shadow Slice — ruling 4

If the creature leaves the battlefield, the exiled card will no longer be encoded on any creature. It will stay exiled.

Shadow Slice — ruling 5

If you want to encode the card with cipher onto a noncreature permanent such as a Keyrune that can turn into a creature, that permanent has to be a creature before the spell with cipher starts resolving. You can choose only a creature to encode the card onto.

Shadow Slice — ruling 6

The copy of the card with cipher is created in and cast from exile.

Shadow Slice — ruling 7

You cast the copy of the card with cipher during the resolution of the triggered ability. Ignore timing restrictions based on the card’s type.

Shadow Slice — ruling 8

If you choose not to cast the copy, or you can’t cast it (perhaps because there are no legal targets available), the copy will cease to exist the next time state-based actions are performed. You won’t get a chance to cast the copy at a later time.

Shadow Slice : règles, fonctionnement et interactions (MTG)

Shadow Slice is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Target opponent loses 3 life. Cipher (Then you may exile this spell card encoded on a creature you control. Whenever that creature deals combat damage to a player, its controller may cast a copy of the encoded card without paying its mana cost.)

How it works : Shadow Slice

Original English text — Wizards of the Coast / Scryfall

#1April 15, 2013· Wizards of the Coast
The spell with cipher is encoded on the creature as part of that spell’s resolution, just after the spell’s other effects. That card goes directly from the stack to exile. It never goes to the graveyard.
No other card is named in this ruling.
#2April 15, 2013· Wizards of the Coast
You choose the creature as the spell resolves. The cipher ability doesn’t target that creature, although the spell with cipher may target that creature (or a different creature) because of its other abilities.
No other card is named in this ruling.
#3April 15, 2013· Wizards of the Coast
If the spell with cipher doesn’t resolve, none of its effects will happen, including cipher. The card will go to its owner’s graveyard and won’t be encoded on a creature.
No other card is named in this ruling.
#4April 15, 2013· Wizards of the Coast
If the creature leaves the battlefield, the exiled card will no longer be encoded on any creature. It will stay exiled.
No other card is named in this ruling.
#5April 15, 2013· Wizards of the Coast
If you want to encode the card with cipher onto a noncreature permanent such as a Keyrune that can turn into a creature, that permanent has to be a creature before the spell with cipher starts resolving. You can choose only a creature to encode the card onto.
No other card is named in this ruling.
#6April 15, 2013· Wizards of the Coast
The copy of the card with cipher is created in and cast from exile.
No other card is named in this ruling.
#7April 15, 2013· Wizards of the Coast
You cast the copy of the card with cipher during the resolution of the triggered ability. Ignore timing restrictions based on the card’s type.
No other card is named in this ruling.
#8April 15, 2013· Wizards of the Coast
If you choose not to cast the copy, or you can’t cast it (perhaps because there are no legal targets available), the copy will cease to exist the next time state-based actions are performed. You won’t get a chance to cast the copy at a later time.
No other card is named in this ruling.
#9April 15, 2013· Wizards of the Coast
The exiled card with cipher grants a triggered ability to the creature it’s encoded on. If that creature loses that ability and subsequently deals combat damage to a player, the triggered ability won’t trigger. However, the exiled card will continue to be encoded on that creature.
No other card is named in this ruling.
#10April 15, 2013· Wizards of the Coast
If another player gains control of the creature, that player will control the triggered ability. That player will create a copy of the encoded card and may cast it.
No other card is named in this ruling.
#11April 15, 2013· Wizards of the Coast
If a creature with an encoded card deals combat damage to more than one player simultaneously (perhaps because some of the combat damage was redirected), the triggered ability will trigger once for each player it deals combat damage to. Each ability will create a copy of the exiled card and allow you to cast it.
No other card is named in this ruling.