Replacement and prevention effects refer to effects that tell the game to replace one action with a different one (prevention effects tell the game to replace damage dealt with nothing). Replacement effects can also alter the way objects enter the battlefield and alter the turn structure by skipping some steps or phases.
In this exercise we’ll talk about replacement effects, prevention effects, and how multiple such effects interact with each other, and other weird game situations they can create. The rules here are not so intuitive and differ from a lot of Magic rules’ conventions, so pay close attention.
Just like a lot of other cases throughout the Magic rules, you can identify replacement and prevention effects by looking for some specific words or phrases:
If you see the word “instead”, you have a replacement effect on you hands
If you see the word “skip”, you have a replacement effect (that replaces a phase/step with nothing)
If an object changes the way it, or other objects enter the battlefield (usually by “[this] enters the battlefield”, and sometimes “with” or “as”), it’s a replacement effect
If you see the word “prevent”, you found yourself a prevention effect (duh!)
For a replacement/prevention effect to apply, it has to be there before the action it replaces/prevents happens. Replacement/prevention effects don’t look back and don’t apply retroactively.
NAP casts Fog during the “declare blockers” step
Fog resolves and creates a prevention effect. When the “combat damage” step starts, all combat damage is prevented.
NAP casts Fog during the “combat damage” step
All combat damage is assigned and dealt, then NAP can cast Fog. By the time Fog resolves, combat damage has already been dealt, and Fog can’t prevent it.
There are no restrictions on casting any spell or activating any ability that would create a replacement/prevention ability. Even though Fog did nothing in the second example, it was cast and resolved legally. Its effect will just end during the cleanup step without ever replacing anything.
If a replacement/prevention effect has a duration, it will stop existing when the duration ends, or when the effect is “used up”. So, if a player casts Abuna's Chant choosing the second option, the effect will end either at the end of turn, or if it gets used up (by preventing 3 points of combat damage and the damage from a Shock during the second main phase, for example). If it has no duration, an effect will wait silently until it’s used up.
There are two things that a replacement effect can’t replace (change “event” to “damage” and “replace” to “prevent” for prevention effects):
An event that it replaced already
An event that didn’t happen
So if, for example, a player is enchanted with two Curse of Bloodlettings, and you cast Shock targeting them, one replacement effect will double the damage (replace 2 damage with 4 damage), then the second one will double the damage (replace 4 with 8). Since the first one already replaced the damage once, it won’t again, so no infinite damage combo there.
If a player casts two Fatigues targeting the same player, that player will skip the next draw step (replace the draw step with nothing), and since the draw step didn’t happen, the second replacement effect can’t skip it, so it will wait for the next draw step to skip.
Replacement effects that are not continuous effects, and change the way their own source resolves, are self-replacement effects (think of Cabal Ritual).
One last (complicated) rule before we move to replacement/prevention effects interacting with each other and other stuff:
Some replacement effects change the way permanents enter the battlefield. To decide what and how will affect the permanent, we look into the future (yes, the future!) and see how the permanent will look on the battlefield, taking into account the replacement effects that would apply to it, as well as continuous effects from its own static abilities, but not from static abilities of other sources. Wow! That’s a mouthful! Just think what would happen if it would enter an empty battlefield, but with all the “enter the battlefield” replacement effects intact. Weird, not intuitive, but once you get it, it’s not so hard.
This is a good point to take a small break and wrap your head around things, because if one replacement effect can be confusing, a bunch of them can really mess things up!
Glad to see you’re back.
Up to this point, we talked about how replacement or prevention effects affect events or damage, but what happens if several replacement/prevention effects should apply to the same thing at the same time? We touched it a little bit in previous examples, but now we’re diving deep into the interaction of replacement and/or prevention effects.
When you have several replacement and/or prevention effects that affect the same object or player at the same, an order for them to apply in should be chosen. The first thing to remember is that, as opposed to most things in Magic, the order is not chosen by the owner of the source, timestamps are ignored, and APNAP order is irrelevant. The order is chosen by the affected player, or the controller of the affected object (or owner, if it has no controller). Truly amazing!
Once the player that gets to decide has been chosen, he should decide based on the following order:
If there’s a self-replacement effect, apply it first
If any of the replacement effects would change under whose control an object would enter the battlefield, choose one of those
If any of the replacement effects would cause an object to become a copy of another object as it enters the battlefield, choose one of those
Choose one of the remaining effects
Go to No. 1, and repeat until all effects either applied, or is no longer applicable
If any event is contained within another event (Making a choice as something enters the battlefield is contained within entering the battlefield, which is contained within creating a token, for example), choose to apply the effect affecting the containing event first, and then the effects that affect the contained event (if we stick to the previous example, replace the token creation, then the entering the battlefield and then the choice).
If applying one effect will cause another effect to become applicable, apply the second one too once it can.
Do you feel ready? If so, you’ll find some questions in the last section of this exercise (wouldn’t be an exercise without it, would it?).
Before you hop on a question answering spree, you can find a less simplified version of the rules here:
Other articles about replacement and prevention effects
For each scenario described in the questions, please state which player decides the ordering of the replacement/prevention effects, and if there are more than one way to order them, describe all of them and how they affect the gamestate.
AP attacks with Boros Swiftblade, Hearthfire Hobgoblin, Battle Hurda and Savannah Lions. NAP doesn’t block, and during the first “Combat Damage” step, they cast Fog. How much combat damage will NAP take?
Can Glistener Elf’s damage to creatures be prevented? To players?
AP attacks with a Skyship Stalker that’s enchanted with Dryad's Favor. NAP controls a Forest, and blocks with a Dragon Hunter that’s enchanted with Street Savvy. Please detail all relevant replacement effects in this scenario and what order they apply in (specify dependencies if there are any).
AP’s Darksteel Colossus dies, and NAP has a Kalitas, Traitor of Ghet. What happens?
AP casts Snapcaster Mage, targeting a Forbid in the graveyard. Later during the turn, AP casts the Forbid using the flashback cost, also paying the buyback cost. What happens?
AP uses the dredge ability of Life from the Loam while Dryad Militant is on the battlefield. What happens?
AP attacks with a Grizzly Bears. NAP blocks with a Drudge Skeletons that’s enchanted with Eel Umbra. During the “Declare Blockers” step, AP casts Shock, targeting Drudge Skeletons, to which NAP responds by regenerating it. What is the gamestate after the “Combat” phase ends?
Blood Moon is on the battlefield and AP plays a Hallowed Fountain. Will AP have to pay life for it to enter the battlefield untapped? Why?
AP is enchanted with two Curse of Bloodlettings. NAP casts Shock targeting AP, to which AP responds by activating Loxodon Anchorite’s ability to prevent the damage. How much damage will be dealt to AP?
AP is at 4 life and NAP is at 2. AP attacks with two Cobblebrutes. NAP doesn’t block, and casts Deflecting Palm during the “Declare Blockers” step, choosing one of the Cobblebrutes. Which player wins the game?