Ashnod's Altar + Gravecrawler + Rooftop Storm

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Overview

With Rooftop Storm active you may cast Gravecrawler from your graveyard for as long as you control a Zombie; after it resolves you can sacrifice that Gravecrawler to Ashnod's Altar for . You can repeat cast → resolve → sacrifice to generate each time only if, at each time you start casting Gravecrawler from the graveyard, you control a (some) Zombie to satisfy Gravecrawler's cast-from-graveyard condition [404.1]. The ruling that “once you've cast Gravecrawler and it's on the stack, it doesn't matter if you lose control of the other Zombie” explains that you only need to control a Zombie at the moment you cast the card, not for the whole time it is on the stack [2011-01-22]. Mandatory additional costs (if any) must still be paid when casting [2011-09-22].

The StackRules & CitationsDetails & Edge Cases

Step 1

Have Rooftop Storm and Ashnod's Altar on the battlefield, and Gravecrawler in your graveyard; make sure you control at least one Zombie permanent before you begin casting (this satisfies Gravecrawler's cast-from-graveyard condition) [404.1].

Step 2

Begin casting Gravecrawler from your graveyard. Rooftop Storm lets you pay rather than Gravecrawler's mana cost, so the mana cost portion is zero; pay any mandatory additional costs if required [2011-09-22].

Step 3

Gravecrawler is a spell on the stack. The ruling means that once the spell is on the stack, it doesn't matter if you lose control of the Zombie that allowed you to cast it [2011-01-22].

Step 4

Allow the Gravecrawler spell to resolve; it becomes a creature on the battlefield under your control. (If it was sacrificed or otherwise moved before it resolved, it will be where it was moved to; sacrifices move a permanent directly to its owner's graveyard [701.14].)

Step 5

Sacrifice that Gravecrawler to Ashnod's Altar: move it from the battlefield to your graveyard and add to your mana pool as Ashnod's Altar's ability instructs. The card is now in your graveyard per the sacrifice action [701.14] and [404.1].

Step 6

If you still control at least one Zombie permanent at the time you start casting Gravecrawler again from your graveyard, Rooftop Storm again allows you to pay and you may repeat the sequence to generate another .

Resolution

You can produce each time you cast Gravecrawler from your graveyard and then sacrifice it to Ashnod's Altar, but you must control at least one Zombie at the moment you cast Gravecrawler from the graveyard for Rooftop Storm to allow the 0 cost; if you have another Zombie that remains on the battlefield (so you meet the cast condition before each cast), you can repeat the sequence indefinitely. The fact you can lose that other Zombie after casting while the spell is on the stack does not retroactively stop that cast [2011-01-22]. Any mandatory additional costs still must be paid when casting [2011-09-22] and sacrificing moves the creature to its owner's graveyard as part of the sacrifice action [701.14].