Hidden Strings

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.

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

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.

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.

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.

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.

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.