tl;dr: If decoy were hidden when it would activate, it wouldn't trigger because hidden enchantments have no effect, and Decoy doesn't specify or require that it triggers while hidden. But instead of triggering when revealed, Decoy triggers when it's destroyed. A destroyed Decoy isn't hidden anymore because only in-play enchantments are hidden, and a destroyed Decoy isn't in play. And it's ok for Decoy to trigger and do stuff even after it's destroyed because Decoy clearly says it does, and if there is a conflict between a card and the rules, the card wins.
Ok, as always, we start by looking up all the easily relevant rules information for [mwcard=MW1E10]Decoy[/mwcard].
Decoy
- If Decoy is destroyed while still unrevealed (e.g. by a Seeking Dispel), its controller still gains the 2 mana.
- If Decoy is countered (e.g. by Nullify), its controller does not gain the 2 mana, because the enchantment never resolved to have its effect.
- If Decoy is discarded from somewhere other than play, such as if it had been planned to be cast with a familiar which then is destroyed, it does not gain its owner 2 mana.
Well, ok. So we know
what the interaction is, but
why does it work?
Let's look at some of the enchantment rules:
There is no limit to the number of different enchantments that can be attached to an object, but each object or zone cannot have more than one enchantment with the same name attached to it at one time. This includes both hidden and revealed enchantments. For example, you cannot attach two Bear Strength spells to the same creature.
This rules passage isn't directly relevant, but it does tell us something important about the general philosophy of enchantments. A hidden [mwcard=MW1E01]Bear Strength[/mwcard] is still a Bear Strength. You can't have two face down Bear Strengths on the same creature, because you can't have two enchantments with the same name attached to the same object, and even when it's hidden the name of a hidden Bear Strength is Bear Strength.
So why doesn't a hidden Bear Strength give a creature Melee +2? Because the rules say so.
Important: Hidden Enchantments have no effect as long as they are hidden!
Ok! But what about [mwcard=MW1E29]Nullify[/mwcard]?
You'll notice that Nullify has rules text that operates while it is hidden. You must reveal Nullify when the creature it's attached to is targeted. That requirement functions even though Nullify is hidden. Being hidden doesn't stop Nullify's rules from working!
So why is Nullify an exception? Because the card says it is. Remember
The Magic Rule on the first page of Chapter One of the rules?
The Magic Rule
Mage Wars is a game of magic and magic spells. Often, these spells will violate the normal rules in some way. In all cases, if there is a conflict between the text on a spell card and these rules, the spell card always takes precedence.
Since Nullify only works if it can trigger while it's hidden, it can!
Ok, so what? Decoy doesn't explicitly require that it functions when it's seeking dispelled. It would still work legally (but be worse) if it only functioned when it's controller chose to reveal it. How does The Magic Rule apply to Decoy?
To answer that question, we need to talk about another card: [mwcard=FWC03]Goblin Bomber[/mwcard]. Goblin Bomber has a very peculiar sort of attack that it can sometimes make when it is destroyed. Normally, an object can't do anything when it's destroyed: a destroyed creature can't make a counterstrike against the attacker that destroyed it, or if a creature burns to death at the start of the upkeep none of its upkeep triggered abilities will later be used.
But Goblin Bomber is different. It can be destroyed
and then make an attack. [mwcard=DNC09]Plague Zombie[/mwcard] and [mwcard=DNE02]Rise Again[/mwcard] also have similar rules. Because these cards
must work after they're destroyed, the cards
do actually work after they're destroyed. That's The Magic Rule in action.
Decoy is one such card. Decoy says, "When this Spell is Destroyed, its controller gains 2 mana." Decoy reaches into our world with glowing gifts on prehensile tendrils from beyond the grave! Spooky!
Er, yes, I was saying....
Let's put all this together then.
A hidden Decoy is still a Decoy, even though it's hidden. When Decoy is destroyed, it's not in play anymore, and we follow the rules for what to do when an enchantment is destroyed: we put it in it's owner's discard pile.
Any spell or object that is destroyed is always placed in its owner’s discard pile
Now, normally we'd ignore a hidden Decoy's abilities because it's hidden, but (and this is the important part) in order for an enchantment to be hidden,
it needs to be in play.
Enchantments are different from all other types of spells in Mage Wars because they are played face down. Your opponent does not know what your enchantment does until you reveal it! A face down enchantment is called a hidden enchantment. You may examine your own hidden enchantments at any time. Enchantments create an object in the arena that remains in play until destroyed.
Since only enchantments that are in play face down are hidden enchantments, once the enchantment is destroyed and leaves play it's not a hidden enchantment anymore. Remember the rule? "Hidden Enchantments have no effect
as long as they are hidden!" Time's up!
* And since it was in play as a Decoy, and left play as a destroyed Decoy, its on-destruction ability triggers! The Decoy's controller gets 2 mana! Hurrah!
This, by the way, also explains why a Nullified Decoy or a discarded Decoy doesn't give any mana. If the Decoy never enters play, its rules never take effect at all, and its trigger condition is never met. And in the absence of specific instructions from the card, the normal game rules apply, and the discarded Decoy just sits like a lump.
Poor lumpy Decoy!
Any questions?
*Ollie Ollie Oxen Free! Last one to the discard pile is a rotten egg!