The rules do not currently cover losing abilities without also setting power and toughness. If you target Lhurgoyf with the Zombie, then Lhurgoyf's power and toughness are undefined.
107.2. If anything needs to use a number that can’t be determined, either as a result or in a calculation, it uses 0 instead.
The problem with losing abilities without also setting power and toughness is that it is ambiguous whether substituting 0 for the result or calculation takes precedent and whether or not the *+1 placeholder in power and toughness box of a creature like Lhurgoyf is used. Depending on the different answers, Lhurgoyf could become 0/0 or 0/1. Until WotC decides to print a card that removes abilities without also setting power and toughness, there is not going to be an official answer on this.
The simple custom card ruling is when a card has a characteristic defining ability that sets its power and/or toughness, then to use the printed value like */*+1 and substitute 0 for *. Note that this is only for characteristic defining abilities and does not apply for all */* cards. There are a few */* cards that don't use characteristic-defining abilities like Primal Clay and Molten Sentry that instead enter with a permanently defined power and toughness and they would still their power and toughness even if the ability was later removed.
Depending on how you want Fire Takes Form to interact, you could reword it similar to Elvish Impersonators so that the power and toughness are defined as it enters rather than with a characteristic defining ability.
The problem with losing abilities without also setting power and toughness is that it is ambiguous whether substituting 0 for the result or calculation takes precedent and whether or not the *+1 placeholder in power and toughness box of a creature like Lhurgoyf is used. Depending on the different answers, Lhurgoyf could become 0/0 or 0/1. Until WotC decides to print a card that removes abilities without also setting power and toughness, there is not going to be an official answer on this.
It's not just a placeholder. If there's no CDA and nothing that sets its P/T, it uses what's printed in the bottom right. It's a */*+1. Since the * can't be determined, it uses 0, and becomes a 0/0+1, or 0/1. Lhurgoyf can lose its CDA if it's in the graveyard and there's a Yixlid Jailer in play.
There is nothing in the comprehensive rules and there are no rulings in gatherer that cover Yixlid Jailer and characteristic defining abilities. And that is not as big of an issue as having undefined power and toughness on the battlefield. It only matters with cards like Reveillark and Corpse Lunge that check or use a value of the power or toughness, and it is when figuring out those cards that we then go to cr107.2 and input 0 for the undefined value.
There is nothing in the comprehensive rules and there are no rulings in gatherer that cover Yixlid Jailer and characteristic defining abilities.
The closest I could find to an official ruling was this [O] ruling from a little more than four years ago. However, it is from 2010, and the rules for CDA's (even those hidden from the CR) may have changed since then.
Private Mod Note
():
Rollback Post to RevisionRollBack
How to use card tags (please use them for everybody's sanity)
[c]Lightning Bolt[/c] -> Lightning Bolt
[c=Lightning Bolt]Apple Pie[/c] -> Apple Pie
Vowels-Only Format Minimum deck size: 60 Maximum number of identical cards: 4 Ban list: Cards whose English names begin with a consonant, Unglued and Unhinged cards, cards involving ante, Ancestral Recall
I remember that the CR did use to state that * values not in play were treated as 0. But that was before the change that made characteristic defining abilities apply in all zones. I do not remember when that change was though. (I think I have back copies of the CR on a disk somewhere, but it would take a while to dig through them to find that change.) With that change, all references to * being treated as 0 were stripped from the CR. Completely removing all of those rules may have been an oversight, but I think it must have been before Yixlid Jailer so there was no scenario in which that rule was any longer needed. That is the logical ruling to make, but the CR does not any longer state that it is so. And even when the CR did cover * being treated as 0, it wasn't the same situation with removed characteristic defining abilities. This may seem overly pedantic, but the CR does explicitly cover most rules interactions, and those that the CR doesn't explicitly cover usually fall under the golden rules that allow card abilities to change or define new rules. The fact that this is not covered is sort of a big deal.
I remember that the CR did use to state that * values not in play were treated as 0. But that was before the change that made characteristic defining abilities apply in all zones. I do not remember when that change was though. (I think I have back copies of the CR on a disk somewhere, but it would take a while to dig through them to find that change.) With that change, all references to * being treated as 0 were stripped from the CR. Completely removing all of those rules may have been an oversight, but I think it must have been before Yixlid Jailer so there was no scenario in which that rule was any longer needed. That is the logical ruling to make, but the CR does not any longer state that it is so. And even when the CR did cover * being treated as 0, it wasn't the same situation with removed characteristic defining abilities. This may seem overly pedantic, but the CR does explicitly cover most rules interactions, and those that the CR doesn't explicitly cover usually fall under the golden rules that allow card abilities to change or define new rules. The fact that this is not covered is sort of a big deal.
I've inquired about the CR being updated to add several uncovered cases (including this one, Council's Judgment, Caged Sun, and others) for a little while now, all to no avail. It's rather unfortunate, but seeing how CR updates go and not covering uncovered cases as time goes on, I think it'd be better just to leave it be and use common sense and/or the four-year-ago ruling to handle the situation.
Private Mod Note
():
Rollback Post to RevisionRollBack
How to use card tags (please use them for everybody's sanity)
[c]Lightning Bolt[/c] -> Lightning Bolt
[c=Lightning Bolt]Apple Pie[/c] -> Apple Pie
Vowels-Only Format Minimum deck size: 60 Maximum number of identical cards: 4 Ban list: Cards whose English names begin with a consonant, Unglued and Unhinged cards, cards involving ante, Ancestral Recall
Why would it need a specific ruling? A creature with it's CDA removed functions the same as a creature that never had a CDA in the first place. It's a vanilla card with */*+1 printed on the bottom right. Since the game can't determine what "*" is when it looks, it uses 0 in its place, as per 107.2.
I will say you guys are making me feel better that this issue confused me in the draft
It was bothering me that I didn't know the answer and had to ask.
I ruled the Fire became a 1/1 Zombie, but the layering was giving me a headache so I want to be sure.
The problem with losing abilities without also setting power and toughness is that it is ambiguous whether substituting 0 for the result or calculation takes precedent and whether or not the *+1 placeholder in power and toughness box of a creature like Lhurgoyf is used. Depending on the different answers, Lhurgoyf could become 0/0 or 0/1. Until WotC decides to print a card that removes abilities without also setting power and toughness, there is not going to be an official answer on this.
The simple custom card ruling is when a card has a characteristic defining ability that sets its power and/or toughness, then to use the printed value like */*+1 and substitute 0 for *. Note that this is only for characteristic defining abilities and does not apply for all */* cards. There are a few */* cards that don't use characteristic-defining abilities like Primal Clay and Molten Sentry that instead enter with a permanently defined power and toughness and they would still their power and toughness even if the ability was later removed.
Depending on how you want Fire Takes Form to interact, you could reword it similar to Elvish Impersonators so that the power and toughness are defined as it enters rather than with a characteristic defining ability.
The closest I could find to an official ruling was this [O] ruling from a little more than four years ago. However, it is from 2010, and the rules for CDA's (even those hidden from the CR) may have changed since then.
[c]Lightning Bolt[/c] -> Lightning Bolt
[c=Lightning Bolt]Apple Pie[/c] -> Apple Pie
Vowels-Only Format
Minimum deck size: 60
Maximum number of identical cards: 4
Ban list: Cards whose English names begin with a consonant, Unglued and Unhinged cards, cards involving ante, Ancestral Recall
I've inquired about the CR being updated to add several uncovered cases (including this one, Council's Judgment, Caged Sun, and others) for a little while now, all to no avail. It's rather unfortunate, but seeing how CR updates go and not covering uncovered cases as time goes on, I think it'd be better just to leave it be and use common sense and/or the four-year-ago ruling to handle the situation.
[c]Lightning Bolt[/c] -> Lightning Bolt
[c=Lightning Bolt]Apple Pie[/c] -> Apple Pie
Vowels-Only Format
Minimum deck size: 60
Maximum number of identical cards: 4
Ban list: Cards whose English names begin with a consonant, Unglued and Unhinged cards, cards involving ante, Ancestral Recall
It was bothering me that I didn't know the answer and had to ask.