Decoy lets you delete this Digimon to save one of your specified Digimon from an opponent's deletion effect. It's insurance for the stack you actually care about, bought with a body you can afford to lose. Note the limits: it only stops effect deletion of the listed type, so battles and non-deletion removal walk right past it.
As printed on cards
Decoy (When your opponent's effects would delete any of your other specified Digimon, by deleting this Digimon, 1 of those Digimon isn't deleted.)
16-18-1. <Decoy> is a keyword effect. When another of your specified Digimon would be deleted by an opponent's effect, by deleting the Digimon with this effect, <Decoy> prevents the Digimon specified by this effect from being deleted. This is shown on cards using text such as <Decoy (Black)>. 16-18-2. <Decoy> is an immediate-type effect that triggers when another of your Digimon specified by this effect would be deleted by an opponent's effect (For details, refer to 15-8-5 "Immediate-Type Effects") 16-18-3. "By deleting this Digimon" in <Decoy> is an optional processing condition. If the content of that condition is executed, the processing for "<Decoy> prevents the Digimon specified by this effect from being deleted" will be mandatory.
Comprehensive Rules §16-18 — official rules page
From Bandai's card rulings. Each links to the card it was asked about.
If an effect such as ＜Decoy＞ prevents deletion when this suspended Digimon would be deleted, can I use this card's ＜All Turns＞ effect to unsuspend this Digimon?
No, you can't. This card's ＜All Turns＞ effect is triggered when one of your Digimon with [Chessmon] is deleted, therefore it won't trigger if such a Digimon isn't deleted.
Ruled Mar 28, 2024 · asked about QueenChessmon (BT13-073)
When this card's ＜When Digivolving＞ effect deletes Digimon in play, can you and your opponent activate ＜Decoy＞, ＜Armor Purge＞, and similar effects to prevent their deletion?
Yes, you can.
Ruled Mar 28, 2024 · asked about Zhuqiaomon (BT8-019)
I try to delete an opponent's Digimon with this card’s ＜When Digivolving＞ effect, and my opponent activates ＜Decoy＞ to prevent the chosen Digimon from being deleted. If the Digimon that activated ＜Decoy＞ is deleted, does the “if no Digimon was deleted by this effect” part of this card's effect still activate?
Yes. The Digimon chosen as a target for this effect is not deleted, and the Digimon that activates ＜Decoy＞ is deleted by the effect of ＜Decoy＞ and not this effect. As such, the condition is satisfied for "if no Digimon was deleted by this effect," and both players will trash the top 5 cards of their decks.
Ruled Mar 28, 2024 · asked about Megidramon (EX2-012)
If a Digimon with this card in its digivolution cards would be deleted other than by one of my effects, then an effect such as ＜Decoy＞ prevents it from being deleted, does this card's inherited effect activate?
Yes, it activates.
Ruled Mar 28, 2024 · asked about X Antibody Proto Form (EX5-070)
Public lines whose key cards carry this keyword, step by step.
15 cards carry this keyword:
Rule text © Bandai, quoted from the official Comprehensive Rules for reference. Combalo is not affiliated with Bandai.
