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-MadMage
Yes, provided you chose to keep the existing Gyruda when the legend rule kicks in (if at all), and assuming you control the existing Gyruda. In general, however, if a legendary permanent named Gyruda, Doom of Depths enters the battlefield under your control while you control another, you won't control both of them by the time you can sacrifice one of them with Altar of Dementia.
If Gyruda enters the battlefield this way, its enters-the-battlefield ability will trigger. Then, if you control two or more legendary permanents named Gyruda, Doom of Depths, the legend rule will bring all but one of them of your choice to the graveyard (C.R. 117.5, 704.5j). And this will happen before—
any player gets priority to activate abilities, including Altar of Dementia's ability (C.R. 117.5, 117.1b), or
triggered abilities (such as the Gyruda ability that just triggered) go on the stack, let alone get to resolve (C.R. 117.5, 117.4).
This means that by the time you get priority to activate Altar of Dementia's ability, you will control at most one legendary permanent named Gyruda, Doom of Depths.
You can't respond to the legend rule, which is a state-based action and happens as soon as possible after the second copy of the same legendary permanent is on the battlefield, before you get priority (the right to cast spells and activate abilites). Note that you can, however, sacrifice the first Gyruda to the Altar before the second enters, and if you don't use the 2 mana immediately, you can keep it in your mana pool until the end of the current step or phase (presumably your main phase)EDIT: Oops, I was thinking of the wrong Altar, no mana involved, cards milled instead, but same principle. In the situation you describe, I see no reason why you would need to wait for the second Gyruda to enter the battlefield to sacrifice the first.
I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
You can't respond to the legend rule, which is a state-based action and happens as soon as possible after the second copy of the same legendary permanent is on the battlefield, before you get priority (the right to cast spells and activate abilites). Note that you can, however, sacrifice the first Gyruda to the Altar before the second enters, and if you don't use the 2 mana immediately, you can keep it in your mana pool until the end of the current step or phase (presumably your main phase)EDIT: Oops, I was thinking of the wrong Altar, no mana involved, cards milled instead, but same principle. In the situation you describe, I see no reason why you would need to wait for the second Gyruda to enter the battlefield to sacrifice the first.
No, you can only respond to the first Gyruda's trigger. You cannot wait for the trigger to resolve and mill cards, because you won't have priority to activate the Altar while the trigger is resolving. Only after it has finished completely and all state based actions (including the legend rule) have been dealt with, and all waiting triggers have been put on the stack, will you get a chance to activate the Altar. By that time, you only have one Gyruda left.
Using card tags is mandatory in the Magic Rulings forum. Please check your private messages to learn how to use them. I've added them for this post.
-MadMage
If Gyruda enters the battlefield this way, its enters-the-battlefield ability will trigger. Then, if you control two or more legendary permanents named Gyruda, Doom of Depths, the legend rule will bring all but one of them of your choice to the graveyard (C.R. 117.5, 704.5j). And this will happen before—
EDIT (Oct. 2, 2021): Add assumption.
and if you don't use the 2 mana immediately, you can keep it in your mana pool until the end of the current step or phase (presumably your main phase)EDIT: Oops, I was thinking of the wrong Altar, no mana involved, cards milled instead, but same principle. In the situation you describe, I see no reason why you would need to wait for the second Gyruda to enter the battlefield to sacrifice the first.RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules
I think maybe he was thinking of Ashnod's Altar.
This is where I'm a little confused. Altar of Dementia is already in play and I cast Gyruda, Doom of Depths from my hand. Everyone mills four cards and I mill four, Gyruda, Doom of Depths is one of those four cards. During that time, can I sacrifice the one in play to Altar of Dementia before bringing the second Gyruda, Doom of Depths onto the battlefield?
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