Creatures
When a creature deals damage in combat, it deals an amount of damage equal to its power. If a creature suffers an amount of damage equalling or exceeding its toughness during a single turn, it has taken lethal damage and is put into the graveyard. If an effect causes a creature's power to drop to less than 0, consider the power 0 for all purposes except that of raising or lowering it. Whenever a creature's toughness is reduced, check that creature to see whether its accumulated damage for the turn equals or exceeds its new toughness; if so, the creature dies. Thus, if a creature's toughness drops to less than 1, it dies automatically. Note that regenerating a creature with a toughness of 0 or less is generally pointless, as regenerating it won't change its toughness.
If an effect changes a creature's power and toughness to specific numbers, this doesn't affect any enchantments, fast effects, or other modifications made to that creature. Only the creature's base power and toughness are affected.
Sue plays Giant Growth ("Target creature gets +3/+3 until end of turn") to make her Durkwood Bears a 7/7 creature. Bob then uses his Sorceress Queen on the Bears, changing it's power and toughness to 0/2 until end of turn. The Bears' normal rating of 4/4 becomes 0/2 which is then modified to 3/5 by the Giant Growth's effect.
A creature's power and toughness are written in an X/Y format, where X is the creature's power, and Y is its toughness. Any modifiers to a creature's power and toughness will be written in the same format (for example, "+2/+1").
Certain creatures are Walls and can't attack. This includes any creature whose card type is "Summon Wall." Walls are treated like other creatures in all other respects.
If you haven't had control of a creature since the start of your most recent turn (counting the current one), then you can't use any of its abilities with TAP in the activation cost, and you can't attack with it. This is referred to as summoning sickness, as it normally applies to a creature you've just summoned, but it applies to all of your creatures regardless of how they came under your control. Note that it's only important whether you've had control of the permanent since the start of your most recent turn, not whether it's been a creature all that time.
If Bob gains control of Sue's Bog Wraith, the Wraith will be "sick" once it comes over to Bob's side, and he won't be able to attack with it right away. As soon as he starts a turn with control of the Wraith, however, it's no longer sick, and he can attack with it that turn.
Sue begins her turn with a Rod of Ruin and an Amulet of Kroog in play. She then plays Titania's Song, which turns all artifacts in play into creatures, with the side effect of wiping out their special abilities. Because she's controlled the Rod of Ruin and the Anulet since the start of her turn, she can attack with them this turn; the fact that they weren't creatures all the time isn't important.
Every creature has a creature type, or category. Certain effects search for creatures of a given type, just as they might search for lands of a given type. A summon card's creature type isn't defined by its name, but by its card type, and sometimes by its card text. On the Hurloon Minotaur, for example, having "Summon Minotaur" as its card type indicates that the creature type is a Minotaur. Artifact creature spells don't have creature types unless their card text says otherwise.
Wall of Spears is an artifact creature, so normally it wouldn't have a creature type. However, it says "Counts as a Wall" in its card text, so it's actually a Wall creature type.
The plurality of creatures isn't important when checking for a creature type match.
Goblin King's ability reads: "All Goblins gain mountainwalk and get +1/+1." Because the plurality isn't important, this ability applies equally to cards that say "Summon Goblin" and those that say "Summon Goblins"
Damage
Certain spells and abilities will deal damage to creatures and/or players. Also, a creature in combat deals an amount of damage equal to its power. Damage that's dealt to a player is simply deducted from his or her life total. Damage dealt to creatures accumulates over the course of the turn. If the damage accumulated by a creature during a single turn is equal to or greater than its toughness, it suffers lethal damage and is put into the graveyard.
Whenever damage is assigned to a creature or player, both players have the opportunity to prevent or redirect the damage during damage prevention (see Damage Prevention).
Regeneration
A creature that's destroyed or suffers damage is put into the graveyard unless it's regenerated, in which case it remains in play. All damage that creature has accumulated that turn is erased. If a creature's been prohibited from regenerating, you can still play effects that would regenerate it, but the regeneration is ignored. A creature card that's already in the graveyard can't be regenerated.
Regenerating a creature taps it, but as a side effect, not as a part of the cost; thus, tapped creatures can still be regenerated. Regenerating a creature in combat removes it from combat.
A creature that's buried or sacrificed can't be regenerated; nothing can prevent that creature from being put into the graveyard. (See Sacrifices.)
Creature Abilities
Certain abilities are standard among creatures. Since these abilities are common, they have specific names and are described in the rulebook. It's important to remember that abilities without a cost are continuous and so are in effect regardless of whether you want them to be.
Regeneration: Several creatures have the innate ability to regenerate. If a creature has regeneration and the ability doesn't say what it regenerates, then it regenerates only itself.
Wall of Brambles reads: "G: Regenerate," so you can regenerate the Brambles for a payment of G, but it's ability can't be used to regenerate other creatures.
Power/Toughness Modifiers: A few creatures can increase their own power and/or toughness. Such abilities typically don't tap the creature with the ability and therefore can be used more than once during a turn. These bonuses usually last only until the end of the turn.
Frozen Shade has the ability: "B: +1/+1 until end of turn." You can increase it's power and toughness as many times in a turn as you can afford, but the power/toughness that's generated as a result lasts only until the end of the turn.
Flying: Creatures without flying can't be assigned to block those with flying. In other words, creatures with flying must be blocked in the air. Creatures with flying can be assigned to block those without the ability, however.
Landwalk: Landwalk is a group of abilities; a creature never has "landwalk," but "islandwalk," "swampwalk," and so on instead. If the defending player controls any lands of the appropriate type, that player can't assign any creatures to block an attacking creature with a landwalk ability.
Lost Soul says "Swampwalk." This means that if you use it to attack your opponent when she has least one swamp in play, it can't be blocked, even by other creatures with swampwalk.
Landhome: Landhome is a group of abilities similar to landwalk. Creatures with landhome can't attack if the defending player controls no lands of the appropriate type. Additionally, any creatures with landhome a player controls are buried if at any time that player controls no lands of the appropriate type.
With one island in play you can cast Sea Serpent (5U), a creature with islandhome. The Serpent remains in play normally, although it can attack only if your opponent controls at least one island as well. If at any time you control no islands (if you lose control of your one island or it's destroyed, for example), you must bury the Serpent immediately.
First Strike: There are actually two damage-dealing steps during the attack. During the first step, all creatures with first strike deal their damage; during the second step, all other creatures deal theirs. Any creature killed by first-strike damage is put into the graveyard before the second damage-dealing step begins.
Bob's Tundra Wolves (1/1, first strike) attack and are blocked by Sue's Llanowar Elves (1/1). During the first-strike damage-dealing step, the Wolves deal 1 damage to the Elves, which is enough to kill the Elves. Since the Elves are in the graveyard during normal damage dealing, the Wolves receive no damage. If Sue had blocked with two Llanowar Elves, the Wolves would've killed only one of them and then in the second step would've received lethal damage themselves.
Trample: Creatures with trample are able to deal damage to a defending player even if blocked. If a creature with trample successfully deals more damage to a creature blocking it than is needed to kill the blocking creature, the excess damage is dealt to the defending player instead of to the blocking creature. If a creature with trample has no blockers to deal damage to (if all of its blockers were killed after blocking it, for example), it deals all of its damage to the defending player.
Your attacking War Mammoth (3/3, trample) is blocked by a Zephyr Falcon (1/1). The Falcon thus receives 3 damage from the Mammoth. Because the Mammoth has trample, the remaining 2 damage is dealt to her opoonent instead of to her Falcon.
Because it's damage in excess of lethal damage that "blows through" to the defending player, preventing damage to a blocking creature will reduce or eliminate the damage that blows through, but it won't save that creature. Also, the damage that blows through is redirected to the defending player and so isn't considered to be successfully dealt to the creature.
A War Mammoth (3/3, trample) is enchanted with Spirit Link, the Spirit Link's controller gets 1 life for every 1 damage dealt by the Mammoth. The Mammoth attacks and is blocked by a Grizzly Bears (2/2). Assuming no damage is prevented, the Mammoth deals 2 damage to the Bears and then goes on to deal 1 damage to the defending player.
Spirit Link's side effect triggers each time damage is dealt: once when damage is dealt to the Bears (2 damage) and again when damage is dealt to the defending player (1 damage). The controller of the Spirit Link thus gains a total of 3 life.
If a creature receives damage from some creatures with trample and some without, all of the trample damage is applied last, maximizing the amount of damage redirected to the defending player.
Sue attacks her opponent with a band of two creatures, a War Mammoth (3/3, trample) and a Mesa Pegasus (1/1, banding). Bob blocks the attack with his Gray Ogre (2/2). During damage dealing, the Ogre is dealt 1 damage from the Pegasus and 3 trample damage from the Mammoth. The Pegasus' damage is applied first, so the Ogre takes another 1 damage before it's killed. The Mammoth's remaining 2 trample damage is redirected to Bob.
Banding: Banding is an ability with two distinct features. The first feature allows you to group creatures into bands so that they can attack your opponent with their combined power. The second feature of banding gives you control over the distribution of combat damage.
An attacking band can contain any number of creatures with banding and up to one creature without it. Once a band's been formed, the creatures within the band attack as a unit. If some of the creatures within the band have special abilities, they retain those abilities, but they don't grant them to the others. Once you've finished declaring attackers, you can't form any new bands or break up existing ones. Creatures with banding aren't required to be part of a band when they attack.
If you band a Benalish Hero (1/1, banding), a Mesa Pegasus (1/1, banding, flying) and a War Mammoth (3/3, trample), they can collectively do 5 damage. Two out of the three creatures have a special ability other than banding: flying or trample. These abilities still apply only to the creatures that possess them. Therefore the Pegasus still flies, but the other's don't; likewise the Mammoth still tramples, but the other's don't.
When a band attacks, the defending player has to either block the creatures as one entity or let all of them through. In other words, any creature assigned to block one member of a band automatically blocks the other creatures in the band as well.
Creatures don't form bands when blocking; they have to block individually. For example, if a single Scryb Sprites (1/1, flying) attacks you, your Mesa Pegasus doesn't allow your creatures without flying to block the Sprites.
The second feature of banding applies only during the damage-dealing step. If combat damage is assigned to a group of creatures that includes at least one creature with banding, whoever controls those creatures decides how the damage is assigned to them. Such a group is typically either an attacking band or a group of creatures that blocked the same attacker and happened to include at least one creature with banding. If both players' creatures receive damage during an attack and both players have at least one creature with banding, then each player gets to distribute the damage his creatures received.
Bob attacks with a Craw Wurm (6/4), Sue blocks with two Durkwood Bears (4/4). After declaring blockers, she uses Helm of Chatzuk to give banding to one of the Bears. Because one of the Bears has banding, Sue - not Bob - decides how damage is distributed to them during damage dealing. The fact that neither one had banding when defense was chosen is irrelevant. Though Sue can pile all damage onto one of her Bears, she instead assigns 3 damage to each of them so that neither one dies; Bob's Wurm takes 8 damage from the Bears and dies.
Protection: A creature with protection is largely immune to spells, effects, and permanents with a given characteristic (usually a certain color). This ability is written as "protection from black," "protection from red," and so on, with the characteristic the creature is shielded from listed at the end.
All forms of protection provide an equivalent set of abilities. For example, if a creature has protection from blue,
- blue creatures can't be assigned to block it;
- all damage dealt to it by blue sources is reduced to 0;
- it can't be the target of blue spells, abilities, or enchantments.
Rampage: Creatures with rampage get bigger and bigger as more creatures are assigned to block them. Rampage always has a certain value, written as "rampage X." If more than one creature is assigned to block an attacking creature with rampage X, that creature gets +X/+X until the end of the turn for each creature assigned to block it after the first.
If three creatures are assigned to block Gorilla Berserkers (2/3, rampage 2), Gorilla Berserkers gets +4/+4 until end of turn (that's +2/+2 for each blocker after the first). This makes Gorilla Berserkers a 6/7 creature until the end of the turn.
Enchantments
There are two types of enchantments: global and local. Global enchantments are simply put into play, but local enchantments are played only on other permanents. Every local enchantment targets a category of permanents. Rather than using a phrase such as "target creature" in its text, however, it defines its target in its card type.
Unholy Strength's card type is "Enchant Creature," so it targets a creature. It's text reads "Enchanted creature gets +2/+1," which means that the creature the spell was cast on increases it's power by 2 and it's toughness by 1 as long as this enchantment is on it.
If the target of a local enchantment becomes invalid or leaves play, the enchantment is buried in its owner's graveyard. Changing control of a permanent doesn't change who controls any enchantments played on that permanent.
Some local enchantments say to play them only, or not to play them, on a subclass of permanents. (For example, an enchant creature might specify that it can't be played on red creatures.) Such stipulations are considered part of the enchantment's targeting requirements and will cause the enchantment to be buried if they're invalidated.
Hot Springs is an enchant land that directs you to play it on a land you control; it can't target lands you don't control. Furthermore, if you play Hot Springs and then lose control of the land it enchants, the enchantment is buried, as it's target becomes illegal.
Normally, global enchantments (that is, those with only "Enchantment" specified as the card type) don't require targets; they're just played in your territory. There are global enchantments, however, that affect specific targets whenever they're activated. Even though these enchantments ask for a target, they're still considered global and are played directly into your territory, not on their target.
Flood is an enchantment that says: "UU: Tap target creature without flying." This card is played directly into your territory, but it affects a creature without flying every time it's activated.