Cranial Insertion: Legally Blocked

Cranial Insertion
Legally Blocked

By Eli Shiffrin, Thijs van Ommen, and Tom Fowler

Welcome back to Cranial Insertion! Today we discuss another section of the rules that is quite difficult to grasp. That section is "500. Legal Attacks and Blocks", which discusses how you determine if a certain set of attacking or blocking assignments is legal. But let's start at the beginning.

The Basic Rules

If no cards are around to complicate matters, the rules for attacking and blocking are quite straightforward. The explanation below is rather technical, but this terminology will make the more troublesome parts of the rules easier to introduce later on.

A Combat Phase in Magic normally consists of five steps: Beginning of Combat, Declare Attackers, Declare Blockers, Combat Damage, and End of Combat. Right at the start of the Declare Attackers step, the attacking player declares an attack. A single attacking assignment is a statement of the form "[creature x] attacks [player y]". Your attack declaration consists of a set of any number of such assignments. This set may be the empty set.

Only creatures that are untapped and that you have controlled since the beginning of the turn may be assigned as attackers. In a duel, you don't get to make a choice about who [player y] is; however, in multiplayer games that use the "Attack Multiple Players Option" (see rule 602), you will get to choose any opponent you want.

Once you have completed your sets of assignments, you use the rules explained in the rest of this article to determine if that set is legal. If you find that the assignment set you wanted to use is illegal, the game rewinds and you have to pick a new set of assignments.

Remember that the entire process of declaring attackers doesn't use the stack. If a player wants to do something before the attack is declared, like tapping a creature so that it can't attack, they'll have to do that in the Beginning of Combat step, before you start making your attack declaration.

The Declare Blockers step functions analogously. A blocking assignment is a statement of the form "[creature x] blocks [creature y]", where [creature x] is an untapped creature you control and [creature y] is a creature that's attacking you. Again, you get to make a set of such assignments. A creature can appear as [creature x] in only one assignment in the set (so each creature can block only one attacker), while there is no such restriction on [creature y] (so an attacking creature can be blocked by several creatures).

Give it flying before it gets blocked,
or the Wight is as good as dead.
Q: My opponent attacks me with Roofstalker Wight, and I block with Loxodon Gatekeeper. After I have done that, my opponent activates the Wight's ability to give it flying, claiming that it is now no longer blocked. I'm pretty sure that shouldn't work.

A: No, indeed it doesn't. The legality of the blocking assignment was only checked once. The creature you declared as a blocker will remain a blocker until end of combat. Your opponent should have given his Wight flying before you declared blockers.

Q: I control two Two-Headed Dragons, and my opponent attacks me with three creatures; let's call them A, B and C. Can I block A and B with one Dragon and B and C with the other?

A: Sure, each Dragon has two heads and can appear as the blocker in two different assignments in your set. The set would contain these assignments:

"Dragon 1 blocks A"
"Dragon 1 blocks B"
"Dragon 2 blocks B"
"Dragon 2 blocks C"


Now it's time to start looking at effects that may interfere with the above rules. There are two classes of such effects: restrictions and requirements. We'll cover restrictions first.

A restriction is a continuous effect that disallows certain (combinations of) assignments. They can apply to attacking or blocking, but in both cases, the rules work the same. Because restrictions can take many forms and are easy enough to understand just by reading them, the technical definitions would only complicate matters so I'll skip them here. All you need to know, is that a set of assignments and a restriction are said to conflict if that set of assignments doesn't follow the restriction. Otherwise, the set of assignments obeys the restriction.

If any restrictions conflict with the set of assignments, then the set of assignments is illegal.

They aren't to likely to end up
blocking, either.
Let's look at some examples of restrictions:

Burning-Tree Bloodscale's :2mana::symr:-ability creates a blocking restriction when it resolves. This restriction conflicts with any assignment set which includes the assignment "[the targeted creature] blocks Burning-Tree Bloodscale".
Manta Ray has an attacking restriction that limits what players can attack, and a blocking restriction that limits what creatures can block it.
Peacekeeper creates the strongest restriction imaginable: "Creatures can’t attack."
Blazing Archon's restriction "Creatures can’t attack you" differs from Peacekeeper's in that it cares about who [player y] is in each of the set's attacking assignments.
Ember Beast's restriction conflicts with any set in which it either attacks or blocks while no other creature is assigned to attack or block.

Q: I control Goblin War Drums and Familiar Ground. Is it true that this makes all my creatures unblockable?

A: Yes, any set of blocking assignments your opponent could come up with will conflict with either the Goblin War Drums's restriction, the Familiar Ground's restriction, or possibly with both. The only blocking assignment set that legal is the empty set: declaring no blockers is your opponent's only option.

If I were two Ember Beasts
I could attack together.
Q: Can two Ember Beasts attack without any another creatures attacking with them?

A: They can, because the legality of the assignment set is only checked for the complete set. That set is

"Ember Beast 1 attacks opponent"
"Ember Beast 2 attacks opponent"

and it's obviously legal, because all restrictions are obeyed.

Some effects function like restrictions, but give you a way out if you pay for it. For example, Ghostly Prison creates a restriction against attacking its controller, but if the attacking player has enough mana to spare, they can break free of the Prison for a turn. However, if the attacking player can't or doesn't want to pay the cost, then the restriction remains a restriction, probably rendering your opponent's attack illegal.

The process for dealing with such costs is described in rules 308.2d-f for attacking and 309.2b-d for blocking. In both cases, after you have come up with an otherwise legal set of attacking or blocking assignments, you first determine the total cost you have to pay, then play mana abilities if you need mana, and finally pay the cost.

Q: I control Elvish Aberration, on which my opponent played Cowed by Wisdom. He has three cards in his hand. Can I get mana from the Elf to pay for his own attacking cost?

A: No. Tapping the creatures that are assigned to attack happens in step 308.2c, so your Elf won't be available as a mana producer when you reach step 308.2e where you get the opporunity to play mana abilities. It would work if the Elf also had vigilance.


Requirements work in the opposite direction of restrictions. They try to force creatures to attack or block instead of keeping them from it.

We define an attacking/blocking requirement as a statement of the form "[set X] must attack/block [set Y] if able". An assignment "[creature x] attacks/blocks [player/creature y]", where [creature x] is in [set X] and [player/creature y] is in [set Y], is said to fulfill the requirement. If an assignment set contains an assignment that fulfills the requirement, then the set as a whole obeys the requirement.

Jason's prayer wasn't answered...
Let's have a look at how an effect breaks down into such requirements in practice.

Into the Fray creates the requirement "[the targeted creature] must attack [any player] if able".
Burning-Tree Bloodscale's :2mana::symg:-ability creates the requirement "[the targeted creature] must block Burning-Tree Bloodscale if able".
Lure creates a seperate requirement on each creature. Each requirement looks like "[this creature] must block [enchanted creature] if able". Any creature not blocking the Lured creature is disobeying its associated requirement.
Goblin Fire Fiend's ability gives the requirement "[any creature] must block Goblin Fire Fiend if able". As far as I know, this is the only card currently which may generate a requirement that has more than one creature in its [set X]; other effects will create seperate requirements like Lure does rather than create a single requirement on a set of creatures.
Nettling Imp's requirement is of the same form as Into the Fray's. Additionally, Nettling Imp's ability will cause the targeted creature to be destroyed if it somehow ends up not attacking. That part doesn't affect the legality of any set of attacking assignments, so we ignore it here.

It may sometimes be impossible to find a set of assignments that obeys all restrictions and requirements. Fortunately, not obeying a requirement doesn't always cause a conflict -- by contrast, not obeying a restriction will always cause a conflict and make the assignment set illegal.

Let's call our set of assignments S. S obeys all its restrictions, but not all its requirements. To determine if S is legal as a declaration of attackers or blockers, we have to check if there exists a different assignment set T. This set T must do a better job of obeying restrictions and requirements than S does. If there is no such set T, then S is good enough, so S is a legal declaration.

"Doing a better job than S" means the following things for the set T:
  1. T obeys all its restrictions;
  2. T obeys more requirements than S.

It's a Wall! What
else would it do?
Q: I control Invasion Plans and attack my opponent with Stalking Tiger. One of his creatures is Razorgrass Screen, so there are now two requirements to block on the Screen, and one on each other creature. Is it legal to have a creature other than the Screen block the Tiger?

A: No, you're stuck stalking the Screen. Blocking with somethings other than the Screen will only fulfill one requirement. This can be improved by blocking with the Screen, which fulfills two requirements, so that's what you have to do.

Remember that the Master Warcrafty part of Invasion Plans doesn't change what's legal and what isn't -- it only changes who makes the decision of how to block.

Q: I attacked my opponent with Stone-Tongue Basilisk (I had threshold), and before he declared blockers, he activated War Cadence with X=0. He claimed he didn't have to block my Basilisk now. Is he right?

A: He is right. Unless the cost is paid, his creatures now have restrictions on them that keep them from blocking. Requirements never force a player to pay for such costs. Your opponent doesn't have to block, even though the cost of 0 mana per creature would be easy to pay.

You are required to come this way.
Q: Dueling Grounds is in play, and I control Viashino Bey and two Viashino Warriors. My opponent has used Bullwhip on the Bey. What are my options?

A: Viashino Bey has a conditional effect: in a set of assignments where the Bey is assigned as an attacker, it creates a requirement on every creature to attack, but in a set where the Bey stays home, the effect does nothing and no requirements are generated. In this scenario, not attacking at all or attacking with a Warrior would mean only the Bullwhip's requirement applies, and this requirement isn't obeyed. Attacking with only the Bey obeys the Bullwhip's requirement as well as the requirement the Bey puts on itself when he attacks, but at the same time this attacking assignment creates two new requirements on the Warriors that aren't obeyed. We only need to count the number of requirements that are obeyed, not the number of requirements that are disobeyed. We find that our only option is to attack with the Bey, which obeys two requirements, while the other options obey none.

Requirements, Schmequirements

In the previous section we saw that if there was no set T that did better than our set S, then set S was legal. However, even if there is such a set T, the set S might still be legal in spite of that. How is this possible? Rule 500.5, the last rule of section 500 we're discussing, limits how far we have to look for the set T. Only a set T that is close enough to the set S can make S illegal.

We only need to consider sets that can be constructed in the following way:
  1. Start with the set S;
  2. Remove any number of assignments from that set;
  3. Add any number of assignment, but only those that fulfill a requirement that S didn't obey. This is your set T.
In other words, the only assignments we have to consider adding when looking for T are those assignments that cause a requirement in S to become obeyed while it wasn't obeyed before.

If you can end up with a set that obeys all its restrictions and more requirements than S, then S is illegal. If you can't, then S is legal.

If S is illegal because we found a set T, then we still don't know whether T is legal or not. All we know is that it came closer to being legal than S. We can rename T to S and start over to find out.

Hugga wugga!
Q: I control two Razorgrass Screens and one Grizzly Bear. My opponent attacks me with Gorilla Berserkers. Can I choose not to block?

A: Yes, not blocking is legal in this situation. If you start out with S as the empty set, then the only sets T you need to consider are those where the Screens try to block something. You don't have to consider any sets T that include assigning your Bear to block the incoming Gorillas, because there is no requirement on the Bear to do something like that. In particular, you don't have to consider blocking with all your creatures, which would be the only other blocking assignment that doesn't conflict with any restrictions.

Q: I control Ekundu Cyclops and Hurloon Minotaur. What are the legal attacks in this situation?

A: Attacking with the Cyclops or with both your creatures are legal choices, because the Cyclops's requirement will obviously be obeyed. Attacking with only the Minotaur isn't legal, because you can find a better set of assignments by also assigning the Cyclops to attack. However, make S the empty set (choosing not to attack), then the Cyclops's conditional ability remains off. Because there are no requirements in this situation, you can't make any changes to S according to the steps described above, so you certainly won't find a set T that would make S illegal. In short, attacking with only the Minotaur is illegal, but anything else goes.

Q: I control the same two creatures, but now I put Errantry on the Minotaur. How does that change matters?

A: Errantry places a restriction on your set of assignments that says that if the Minotaur wants to attack, he must do so alone. This means that attacking with both creatures is now also illegal as it conflicts with the new restriction, but nothing else changes.

And the last question
is for the cat.
Q: I attack with a Krosan Vorine and a Grizzly Bears. The defending player has two untapped creatures: a Watchdog and a Hill Giant. I target the Hill Giant with the Vorine's provoke ability. How can my opponent block?

A: The only restriction is the Vorine's, which prohibits the Dog and the Giant from ganging up on it. There are also two requirements at work here. For whatever assignment set S we choose, we can construct the set T by first removing anything in S, then adding "Giant blocks Vorine" and "Watchdog blocks Bear". This is the only set that obeys both requirements, so it's the only legal option.

In a Nutshell

Here you find the most important info from this article condensed to a few lines. That doesn't necessarily make it easier to understand, though. Smile

A set S of assignments is legal if and only if (a) it doesn't conflict with any of its restrictions, and (b) there is no set of assignments T that meets the following conditions:
  1. T obeys all its restrictions;
  2. T obeys more requirements than S;
  3. Each assignment that is in T but not is S satisfies some requirement in S that isn't obeyed in S.

Well, that was that. I doubt this answered all your questions, so keep them coming at [email][email protected][/email]!

-Thijs van Ommen, The Netherlands


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