I am both selfish and instinctive. I value growth and community, as long as they favour my own objectives; I enjoy nature, and I particularly enjoy watching parts of nature die. At best, I am resilient and tenacious; at worst, I'm uncontrollable and destructive.
No you cannot. Before you pay all of the costs of the spell, you have to produce the mana. So you would have to sac the creature to the Altar before you get the chance to tap it for convoke.
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Questions don't have to make sense, but answers do.
I am both selfish and instinctive. I value growth and community, as long as they favour my own objectives; I enjoy nature, and I particularly enjoy watching parts of nature die. At best, I am resilient and tenacious; at worst, I'm uncontrollable and destructive.
601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Casting a spell follows the steps listed below, in order. If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the casting of the spell is illegal; the game returns to the moment before that spell started to be cast (see rule 717, “Handling Illegal Actions&rdquo. Announcements and payments can’t be altered after they’ve been made.
601.2a The player announces that he or she is casting the spell. That card (or that copy of a card) moves from where it is to the stack. It becomes the topmost object on the stack. It has all the characteristics of the card (or the copy of a card) associated with it, and that player becomes its controller. The spell remains on the stack until it’s countered, it resolves, or an effect moves it elsewhere.
601.2b If the spell is modal the player announces the mode choice (see rule 700.2). If the player wishes to splice any cards onto the spell (see rule 702.45), he or she reveals those cards in his or her hand. If the spell has alternative or additional costs that will be paid as it’s being cast such as buyback, kicker, or convoke costs (see rules 117.8 and 117.9), the player announces his or her intentions to pay any or all of those costs (see rule 601.2e). A player can’t apply two alternative methods of casting or two alternative costs to a single spell. If the spell has a variable cost that will be paid as it’s being cast (such as an {X} in its mana cost; see rule 107.3), the player announces the value of that variable. If a cost that will be paid as the spell is being cast includes hybrid mana symbols, the player announces the nonhybrid equivalent cost he or she intends to pay. If a cost that will be paid as the spell is being cast includes Phyrexian mana symbols, the player announces whether he or she intends to pay 2 life or the corresponding colored mana cost for each of those symbols. Previously made choices (such as choosing to cast a spell with flashback from a graveyard or choosing to cast a creature with morph face down) may restrict the player’s options when making these choices.
601.2c The player announces his or her choice of an appropriate player, object, or zone for each target the spell requires. A spell may require some targets only if an alternative or additional cost (such as a buyback or kicker cost), or a particular mode, was chosen for it; otherwise, the spell is cast as though it did not require those targets. If the spell has a variable number of targets, the player announces how many targets he or she will choose before he or she announces those targets. The same target can’t be chosen multiple times for any one instance of the word “target” on the spell. However, if the spell uses the word “target” in multiple places, the same object, player, or zone can be chosen once for each instance of the word “target” (as long as it fits the targeting criteria). If any effects say that an object or player must be chosen as a target, the player chooses targets so that he or she obeys the maximum possible number of such effects without violating any rules or effects that say that an object or player can’t be chosen as a target. The chosen players, objects, and/or zones each become a target of that spell. (Any abilities that trigger when those players, objects, and/or zones become the target of a spell trigger at this point; they’ll wait to be put on the stack until the spell has finished being cast.)
Example: If a spell says “Tap two target creatures,” then the same creature can’t be chosen twice; the spell requires two different legal targets. A spell that says “Destroy target artifact and target land,” however, can target the same artifact land twice because it uses the word “target” in multiple places.
601.2d If the spell requires the player to divide or distribute an effect (such as damage or counters) among one or more targets, the player announces the division. Each of these targets must receive at least one of whatever is being divided.
601.2e The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be {0}. It can’t be reduced to less than {0}. Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes “locked in.” If effects would change the total cost after this time, they have no effect.
601.2f If the total cost includes a mana payment, the player then has a chance to activate mana abilities (see rule 605, “Mana Abilities&rdquo. Mana abilities must be activated before costs are paid.
601.2g The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can’t be paid.
Example: You cast Altar’s Reap, which costs {1}{B} and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost {1} less to cast. Because a spell’s total cost is “locked in” before payments are actually made, you pay {B}, not {1}{B}, even though you’re sacrificing the Familiar.
601.2h Once the steps described in 601.2a–g are completed, the spell becomes cast. Any abilities that trigger when a spell is cast or put onto the stack trigger at this time. If the spell’s controller had priority before casting it, he or she gets priority.
Mana can be generated before starting to cast the spell or in 601.2f. Both the convoke payment and the mana payment of the costs happen in 601.2g.
702.49. Convoke
702.49a Convoke is a static ability that functions while the spell with convoke is on the stack. “Convoke” means “As an additional cost to cast this spell, you may tap any number of untapped creatures you control. Each creature tapped this way reduces the cost to cast this spell by {1} or by one mana of any of that creature’s colors.” Using the convoke ability follows the rules for paying additional costs in rules 601.2b and 601.2e–g.
What nate said was correct, but perhaps a bit incomplete. In greater detail, here's the process of casting Hour of Reckoning
1) You start casting Hour of Reckoning 2) You announce that you will be convoking (including how many times and which colors of mana) 3) You determine the total cost of the spell 4) You produce mana 5) You pay the total cost of the spell
So, let's say you have a single colorless creature you intend to use to reduce the cost. Let's walk through the steps:
1) You start casting it 2) You announce that you're convoking with one creature to reduce the cost by one generic mana 3) You determine the total cost to be "3WWW, tap an untapped creature you control" 4) You produce mana, by sacrificing your only creature to ashnod's alter, and tapping four plains. You now have 2WWWW in your mana pool 5) You try to pay the total cost. The 3WWW you've got in your mana pool all ready to go, but you don't have a creature to tap.
601.2e The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be {0}. It can't be reduced to less than {0}. Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes "locked in." If effects would change the total cost after this time, they have no effect.
601.2f If the total cost includes a mana payment, the player then has a chance to activate mana abilities (see rule 605, "Mana Abilities"). Mana abilities must be activated before costs are paid.
601.2g The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can't be paid. Example: You cast Altar's Reap, which costs {1}{B} and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost {1} less to cast. Because a spell's total cost is "locked in" before payments are actually made, you pay {B}, not {1}{B}, even though you're sacrificing the Familiar.
2f and 2g are the relevant rules here. We lock in the costs in 2e, but before we move on to paying the costs in 2g, we get the chance to activate mana abilities like the Altar. Then in 2g, we can pay the costs (which include tapping the creatures for the convoke ability). If you saced a creature for mana during 2f, it's no longer around in 2g to be tapped. So you can either sac the creature to get the mana or tap it for convoke, never both.
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Questions don't have to make sense, but answers do.
I am both selfish and instinctive. I value growth and community, as long as they favour my own objectives; I enjoy nature, and I particularly enjoy watching parts of nature die. At best, I am resilient and tenacious; at worst, I'm uncontrollable and destructive.
So, let's say you have a single colorless creature you intend to use to reduce the cost or pay for it. Let's walk through the steps:
1) You start casting it 2) You announce that you're convoking with one creature to reduce the cost by one generic mana 3) You determine the total cost to be "3WWW, tap an untapped creature you control" 4) You produce mana, by sacrificing your only creature to ashnod's alter, and tapping four plains. You now have 2WWWW in your mana pool 5) You try to pay the total cost. The 3WWW you've got in your mana pool all ready to go, but you don't have a creature to tap.
6) You undo steps 1-5 above. The game "rewinds" to before step 1, and you may try again (the creature wasn't sacrificed, the lands weren't tapped, Hour of Reckoning is in your hand, etc)
Note: in some case certain actions are irreversible
717.1. If a player realizes that he or she can’t legally take an action after starting to do so, the entire action is reversed and any payments already made are canceled. No abilities trigger and no effects apply as a result of an undone action. If the action was casting a spell, the spell returns to the zone it came from. The player may also reverse any legal mana abilities activated while making the illegal play, unless mana from them or from any triggered mana abilities they triggered was spent on another mana ability that wasn’t reversed. Players may not reverse actions that moved cards to a library, moved cards from a library to any zone other than the stack, or caused a library to be shuffled.
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. Announcements and payments can’t be altered after they’ve been made.
