If I have 30 life and, while having a Scourge of the Skyclaves in my graveyard, mutate Nethroi, Apex of Death, does targeting Scourge allow me to return 20 power of creatures rather than 10?
If yes, would causing the highest life total to decrease or exiling Scourge with the mutate trigger on the stack cause the mutate ability to fizzle in that scenario?
Under C.R. 107.1b, if "a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect ... sets to a specific value ... a creature's power and/or toughness", among other exceptions not relevant here. For the purposes of C.R. 107.1b, Scourge of the Skyclaves is not a "creature" while it's not on the battlefield (see also C.R. 109.2), so that if "20 minus the highest life total among players" would be less than 0, that number is treated as 0 while Scourge of the Skyclaves is in your graveyard.
Thus, for the purposes of Nethroi's last ability, Scourge of the Skyclaves has a power of 0 while it's in your graveyard.
EDIT: Struck out after comment 4 was posted.
EDIT (Jan. 9, 2022): Note that with Zendikar Rising, C.R. 107.1b was changed in relevant part to refer to setting the power and/or toughness of a "creature or creature card", so that the struck-out text in this comment is obsolete anyway.
"Most of the time, the Magic game uses only positive numbers and zero. You can’t choose a negative number, deal negative damage, gain negative life, and so on. However, it’s possible for a game value, such as a creature’s power, to be less than zero. If a calculation or comparison needs to use a negative value, it does so. If a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect doubles or sets to a specific value a player’s life total or a creature’s power and/or toughness." (107.1b, emphasis added)
Negative numbers are used until an effect would assume a negative value. Nethroi is able to use the negative value for Scourge's power for the calculation (summing) and comparison (less than 10), but it is not the kind of effect that takes on a numerical value for its result at all. Keep in mind that the "with total power 10 or less" descriptor is a targeting requirement, so it's not an effect, only the movement of those cards is.
In short, this rule is telling us to let a magnitude effect, at the point of application after any math, have only nonnegative magnitude, except for those life total and creature stat cases.
Private Mod Note
():
Rollback Post to RevisionRollBack
Epic banner by Erasmus of æтђєг.
Awesome avatar provided by Krashbot @ [Epic Graphics].
If yes, would causing the highest life total to decrease or exiling Scourge with the mutate trigger on the stack cause the mutate ability to fizzle in that scenario?
No.
Under C.R. 107.1b, if "a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect ... sets to a specific value ... a creature's power and/or toughness", among other exceptions not relevant here. For the purposes of C.R. 107.1b, Scourge of the Skyclaves is not a "creature" while it's not on the battlefield (see also C.R. 109.2), so that if "20 minus the highest life total among players" would be less than 0, that number is treated as 0 while Scourge of the Skyclaves is in your graveyard.
Thus, for the purposes of Nethroi's last ability, Scourge of the Skyclaves has a power of 0 while it's in your graveyard.
EDIT: Struck out after comment 4 was posted.
EDIT (Jan. 9, 2022): Note that with Zendikar Rising, C.R. 107.1b was changed in relevant part to refer to setting the power and/or toughness of a "creature or creature card", so that the struck-out text in this comment is obsolete anyway.
https://mobile.twitter.com/WotC_Matt/status/1302725023614992386
"Most of the time, the Magic game uses only positive numbers and zero. You can’t choose a negative number, deal negative damage, gain negative life, and so on. However, it’s possible for a game value, such as a creature’s power, to be less than zero. If a calculation or comparison needs to use a negative value, it does so. If a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect doubles or sets to a specific value a player’s life total or a creature’s power and/or toughness." (107.1b, emphasis added)
Negative numbers are used until an effect would assume a negative value. Nethroi is able to use the negative value for Scourge's power for the calculation (summing) and comparison (less than 10), but it is not the kind of effect that takes on a numerical value for its result at all. Keep in mind that the "with total power 10 or less" descriptor is a targeting requirement, so it's not an effect, only the movement of those cards is.
In short, this rule is telling us to let a magnitude effect, at the point of application after any math, have only nonnegative magnitude, except for those life total and creature stat cases.
Awesome avatar provided by Krashbot @ [Epic Graphics].