Do you have to name a source that's on the battlefield or stack for Deflecting Palm?
For a very relevant example, if my opponent casts Ad Nauseam and then casts and resolves Angel's Grace in response, can I cast Deflecting Palm that will be naming Lightning Storm in response to Ad Nauseam?
No, you won't be able to do that, because the rules for choosing a source of damage don't allow you to make such a choice (and you must choose it at the time Deflecting Palm resolves). For one thing, intuitively speaking, you wouldn't be able to distinguish between different Lightning Storms.
Quote from Comp. rules »
609.7a If an effect requires a player to choose a source of damage, he or she may choose a permanent; a spell on the stack (including a permanent spell); any object referred to by an object on the stack, by a replacement or prevention effect that’s waiting to apply, or by a delayed triggered ability that’s waiting to trigger (even if that object is no longer in the zone it used to be in); or, for certain casual variant games, a face-up card in the command zone. A source doesn’t need to be capable of dealing damage to be a legal choice. The source is chosen when the effect is created. If the player chooses a permanent, the effect will apply to the next damage dealt by that permanent, regardless of whether it’s combat damage or damage dealt as the result of a spell or ability. If the player chooses a permanent spell, the effect will apply to any damage dealt by that spell and any damage dealt by the permanent that spell becomes when it resolves.
PS: As the set Deflecting Palm is in (KTK) has already been released, this actually should be in the primary Rulings forum, since it's no longer a "rumored card."
Edit:
PPS: Also note that you will be able to cast Deflecting Palm in response to Lightning Storm and achieve the result you want if it resolves (I suppose you might be worried about cards like Force of Will).
Well, you can't because the source of damage -- Lightning Storm spell on stack -- doesn't exist at this point. There's just a Lightning Storm card and even if you could name THAT, it would become a different object after casting. Any zone change means that a new object is created with sole exception of moving from stack to battlefield.
Plus, even if this would work, you'd still have to pick a SPECIFIC Lightning Storm card, so your opponent could cast a different one.
Theoretically, you can choose practically anything in a public zone (so not hand or library, because you either don't know what possible sources are there or it would be hard to verify) for Deflecting Palm, even though choosing things that are not on battlefield or stack will seldom do anything. I guess you could for example pick Gempalm Incinerator in a graveyard to deal with the damage from its cycling ability.
But why not simply cast Deflecting Palm in response to Lightning Storm itself? At that point, you can select it as the source and Deflecting Palm would work as expected.
Well, you can't because the source of damage -- Lightning Storm spell on stack -- doesn't exist at this point. There's just a Lightning Storm card and even if you could name THAT, it would become a different object after casting. Any zone change means that a new object is created with sole exception of moving from stack to battlefield.
Plus, even if this would work, you'd still have to pick a SPECIFIC Lightning Storm card, so your opponent could cast a different one.
Theoretically, you can choose practically anything in a public zone (so not hand or library, because you either don't know what possible sources are there or it would be hard to verify) for Deflecting Palm, even though choosing things that are not on battlefield or stack will seldom do anything. I guess you could for example pick Gempalm Incinerator in a graveyard to deal with the damage from its cycling ability.
But why not simply cast Deflecting Palm in response to Lightning Storm itself? At that point, you can select it as the source and Deflecting Palm would work as expected.
Ad Nauseam holds a zillion Pact of Negations after Ad Nauseam resolves. Guess Deflecting Palm won't hose the combo, then.
Do you have to name a source that's on the battlefield or stack for Deflecting Palm?
For a very relevant example, if my opponent casts Ad Nauseam and then casts and resolves Angel's Grace in response, can I cast Deflecting Palm that will be naming Lightning Storm in response to Ad Nauseam?
You don't "name" a source, you choose one. To choose something, it has to exist. Which Lightning Storm do you mean? None, because there isn't one anywhere.
CR 609.7 says "If an effect requires a player to choose a source of damage, he or she may choose a permanent; a spell on the stack (including a permanent spell); any object referred to by an object on the stack, by a replacement or prevention effect that’s waiting to apply, or by a delayed triggered ability that’s waiting to trigger (even if that object is no longer in the zone it used to be in)"
It looks complex but in practice this is just precisely what, intuitively, a player could choose as a source of impending damage. To choose a Lightning Storm, you must wait until one is either on the stack, or is a card somewhere which some spell or ability on the stack is referring to. If you would describe how a Lightning Storm is going to enter the scenario you posted, we could be concrete about this.
The above contradicts some information from Marek14.
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For a very relevant example, if my opponent casts Ad Nauseam and then casts and resolves Angel's Grace in response, can I cast Deflecting Palm that will be naming Lightning Storm in response to Ad Nauseam?
PS: As the set Deflecting Palm is in (KTK) has already been released, this actually should be in the primary Rulings forum, since it's no longer a "rumored card."
Edit:
PPS: Also note that you will be able to cast Deflecting Palm in response to Lightning Storm and achieve the result you want if it resolves (I suppose you might be worried about cards like Force of Will).
Plus, even if this would work, you'd still have to pick a SPECIFIC Lightning Storm card, so your opponent could cast a different one.
Theoretically, you can choose practically anything in a public zone (so not hand or library, because you either don't know what possible sources are there or it would be hard to verify) for Deflecting Palm, even though choosing things that are not on battlefield or stack will seldom do anything. I guess you could for example pick Gempalm Incinerator in a graveyard to deal with the damage from its cycling ability.
But why not simply cast Deflecting Palm in response to Lightning Storm itself? At that point, you can select it as the source and Deflecting Palm would work as expected.
Ad Nauseam holds a zillion Pact of Negations after Ad Nauseam resolves. Guess Deflecting Palm won't hose the combo, then.
You don't "name" a source, you choose one. To choose something, it has to exist. Which Lightning Storm do you mean? None, because there isn't one anywhere.
CR 609.7 says "If an effect requires a player to choose a source of damage, he or she may choose a permanent; a spell on the stack (including a permanent spell); any object referred to by an object on the stack, by a replacement or prevention effect that’s waiting to apply, or by a delayed triggered ability that’s waiting to trigger (even if that object is no longer in the zone it used to be in)"
It looks complex but in practice this is just precisely what, intuitively, a player could choose as a source of impending damage. To choose a Lightning Storm, you must wait until one is either on the stack, or is a card somewhere which some spell or ability on the stack is referring to. If you would describe how a Lightning Storm is going to enter the scenario you posted, we could be concrete about this.
The above contradicts some information from Marek14.
Awesome avatar provided by Krashbot @ [Epic Graphics].