Q: I want to know if Elspeth, Knight-Errant gives the indestructibility to your permanents on the field at the time of activation of its last effect, or if the effect works on all permanents you play after the activation of the effect.
–James, Kingston, NY, USA
A: From the Magic Rules Corner:
When Elspeth, Knight-Errant's –8 ability resolves, it creates an effect that lasts until the end of the game and applies to all artifacts, creatures, enchantments, and lands you control at any point for the rest of the game, regardless of when they come under your control. It stops applying to such permanents that you lose control of for any reason, and it continues to apply even if Elspeth leaves play. In other words, the set of permanents that it applies to isn't "locked in" when the ability resolves.
You may note that this is different from cards such as Overrun, which applies only to creatures that you control when it resolves.
The reason for this is that Overrun changes the characteristics of permanents. (The characteristics are name, mana cost, color, card type, subtype, supertype, expansion symbol, rules text, abilities, power, toughness, and loyalty; anything else that's true about an object in the game isn't a characteristic.) Elspeth, on the other hand, states something that is now true about these permanents you control without altering their characteristics.
Being indestructible isn't a characteristic, nor is it an ability. Some permanents have the ability "[This] is indestructible," which is rules text and therefore is a characteristic, but Elspeth's –8 ability doesn't grant any abilities and thus doesn't alter characteristics. If a creature that's indestructible because of Elspeth loses all abilities, it will still be indestructible.
To illustrate this, consider two hypothetical instants. One says, "Until end of turn, creatures can't attack." The other says, "All creatures gain defender until end of turn." These will, under most circumstances, do the same thing, but they're different in terms of how they affects creatures that come into play (or become creatures) after they resolve. The first one states something about creatures but doesn't alter their characteristics, so it would apply to all creatures, even those that came into play after it resolved. The second grants an ability to all creatures, which is a characteristic, so it would "look at" all the creatures in play when it resolves and cause them to gain defender. Any creatures that came into play or became creatures after the spell resolved wouldn't have defender and would be able to attack (although if they just came into play, they wouldn't be able to attack unless they had haste, as normal).
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