A replacement effect is a type of continuous effect that watches for one event to happen, and replaces it entirely with another event. The wording of these effects use "instead", "As <something happens>, ...", or "<Some permanent> comes into play..." to indicate they are replacement effects.
From the Comprehensive Rules (Khans of Tarkir)
- 614. Replacement Effects
- 614.1. Some continuous effects are replacement effects. Like prevention effects (see rule 615), replacement effects apply continuously as events happen -- they aren't locked in ahead of time. Such effects watch for a particular event that would happen and completely or partially replace that event with a different event. They act like "shields" around whatever they're affecting.
- 614.1a. Effects that use the word "instead" are replacement effects. Most replacement effects use the word "instead" to indicate what events will be replaced with other events.
- 614.1b. Effects that use the word "skip" are replacement effects. These replacement effects use the word "skip" to indicate what events, steps, phases, or turns will be replaced with nothing.
- 614.1c. Effects that read "[This permanent] enters the battlefield with . . . ," "As [this permanent] enters the battlefield . . . ," or "[This permanent] enters the battlefield as . . . " are replacement effects.
- 614.1d. Continuous effects that read "[This permanent] enters the battlefield . . ." or "[Objects] enter the battlefield . . ." are replacement effects.
- 614.1e. Effects that read "As [this permanent] is turned face up . . . ," are replacement effects.
- 614.2. Some replacement effects apply to damage from a source. See rule 609.7.
- 614.3. There are no special restrictions on casting a spell or activating an ability that generates a replacement effect. Such effects last until they're used up or their duration has expired.
- 614.4. Replacement effects must exist before the appropriate event occurs -- they can't "go back in time" and change something that's already happened. Spells or abilities that generate these effects are often cast or activated in response to whatever would produce the event and thus resolve before that event would occur.
Example: A player can activate an ability to regenerate a creature in response to a spell that would destroy it. Once the spell resolves, though, it's too late to regenerate the creature.
- 614.5. A replacement effect doesn't invoke itself repeatedly; it gets only one opportunity to affect an event or any modified events that may replace it.
Example: A player controls two permanents, each with an ability that reads "If a creature you control would deal damage to a creature or player, it deals double that damage to that creature or player instead." A creature that normally deals 2 damage will deal 8 damage -- not just 4, and not an infinite amount.
- 614.6. If an event is replaced, it never happens. A modified event occurs instead, which may in turn trigger abilities. Note that the modified event may contain instructions that can't be carried out, in which case the impossible instruction is simply ignored.
- 614.7. If a replacement effect would replace an event, but that event never happens, the replacement effect simply doesn't do anything.
- 614.7a If a source would deal 0 damage, it does not deal damage at all. Replacement effects that would increase the damage dealt by that source, or would have that source deal that damage to a different object or player, have no event to replace, so they have no effect.
- 614.8. Regeneration is a destruction-replacement effect. The word "instead" doesn't appear on the card but is implicit in the definition of regeneration. "Regenerate [permanent]" means "The next time [permanent] would be destroyed this turn, instead remove all damage marked on it and tap it. If it's an attacking or blocking creature, remove it from combat." Abilities that trigger from damage being dealt still trigger even if the permanent regenerates. See rule 701.12.
- 614.9. Some effects replace damage dealt to one creature, planeswalker, or player with the same damage dealt to another creature, planeswalker, or player; such effects are called redirection effects. If either creature or planeswalker is no longer on the battlefield when the damage would be redirected, or is no longer a creature or planeswalker when the damage would be redirected, the effect does nothing. If damage would be redirected to or from a player who has left the game, the effect does nothing.
- 614.10. An effect that causes a player to skip an event, step, phase, or turn is a replacement effect. "Skip [something]" is the same as "Instead of doing [something], do nothing." Once a step, phase, or turn has started, it can no longer be skipped -- any skip effects will wait until the next occurrence.
- 614.10a Anything scheduled for a skipped step, phase, or turn won't happen. Anything scheduled for the "next" occurrence of something waits for the first occurrence that isn't skipped. If two effects each cause a player to skip his or her next occurrence, that player must skip the next two; one effect will be satisfied in skipping the first occurrence, while the other will remain until another occurrence can be skipped.
- 614.10b Some effects cause a player to skip a step, phase, or turn, then take another action. That action is considered to be the first thing that happens during the next step, phase, or turn to actually occur.
- 614.11. Some effects replace card draws. These effects are applied even if no cards could be drawn because there are no cards in the affected player's library.
- 614.11a If an effect replaces a draw within a sequence of card draws, all actions required by the replacement are completed, if possible, before resuming the sequence.
- 614.11b If an effect would have a player both draw a card and perform an additional action on that card, and the draw is replaced, the additional action is not performed on any cards that are drawn as a result of that replacement effect.
- 614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c - d.) Such effects may come from the permanent itself if they affect only that permanent (as opposed to a general subset of permanents that includes it). They may also come from other sources. To determine which replacement effects apply and how they apply, check the characteristics of the permanent as it would exist on the battlefield, taking into account replacement effects that have already modified how it enters the battlefield (see rule 616.1), continuous effects generated by the resolution of spells or abilities that changed the permanent's characteristics on the stack (see rule 400.7a), and continuous effects from the permanent's own static abilities, but ignoring continuous effects from any other source that would affect it.
Example: Voice of All says "As Voice of All enters the battlefield, choose a color" and "Voice of All has protection from the chosen color." An effect creates a token that's a copy of Voice of All. As that token is put onto the battlefield, its controller chooses a color for it.
Example: Yixlid Jailer says "Cards in graveyards lose all abilities." Scarwood Treefolk says "Scarwood Treefolk enters the battlefield tapped." A Scarwood Treefolk that's put onto the battlefield from a graveyard enters the battlefield tapped.
Example: Orb of Dreams is an artifact that says "Permanents enter the battlefield tapped." It won't affect itself, so Orb of Dreams enters the battlefield untapped.
- 614.12a If a replacement effect that modifies how a permanent enters the battlefield requires a choice, that choice is made before the permanent enters the battlefield.
- 614.13. An effect that modifies how a permanent enters the battlefield may cause other objects to change zones.
- 614.13a When applying an effect that modifies how a permanent enters the battlefield, you can't make a choice that would cause that permanent to go to a different zone and not enter the battlefield.
Example: Sutured Ghoul says, in part, "As Sutured Ghoul enters the battlefield, exile any number of creature cards from your graveyard." If Sutured Ghoul enters the battlefield from your graveyard, Sutured Ghoul itself can't be exiled by this replacement effect.
- 614.13b The same object can’t be chosen to change zones more than once when applying replacement effects that modify how a single permanent enters the battlefield.
Example: Jund (a plane card) says, "Whenever a player casts a black, red, or green creature spell, it gains devour 5." A player controls Runeclaw Bear and casts Thunder-Thrash Elder, a red creature with devour 3. As Thunder-Thrash enters the battlefield, its controller can choose to sacrifice Runeclaw Bear when applying the devour 3 effect or when applying the devour 5 effect, but not both. Thunder-Thrash Elder will enter the battlefield with zero, three, or five +1/+1 counters, depending on this choice.
- 614.14. An object may have one ability printed on it that generates a replacement effect which causes one or more cards to be exiled, and another ability that refers either to "the exiled cards" or to cards "exiled with [this object]." These abilities are linked: the second refers only to cards in the exile zone that were put there as a direct result of the replacement event caused by the first. If another object gains a pair of linked abilities, the abilities will be similarly linked on that object. They can't be linked to any other ability, regardless of what other abilities the object may currently have or may have had in the past. See rule 607, "Linked Abilities."
- 614.15. Some replacement effects are not continuous effects. Rather, they are an effect of a resolving spell or ability that replace part or all of that spell or ability's own effect(s). Such effects are called self-replacement effects. The text creating a self-replacing effect is usually part of the ability whose effect is being replaced, but the text can be a separate ability, particularly when preceded by an ability word. When applying replacement effects to an event, self-replacement effects are applied before other replacement effects.