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Does this trick work?
If this works hinges on the question if they rule the phasms alternate form ability qualifies for Assume Supernatural Ability. That feat says:
You learn to use a single supernatural ability of another kind of creature while assuming its form through a polymorph self spell or a similar effect.
To quote from @Chemus answer:
Interactions between 3rd and 3.5 form altering abilities are, therefore, to put it in technical terms, an awful mess. This highlights why some players will say that most form altering magic that lets a creature appear to be another to meet the 'polymorph selfspell or a similar effect' metric of the Assume Supernatural Ability feat, and others adhere to only converting the words polymorph self to polymorph.
If your DM decides this is similar enough, it will work. However, there are a lot of differences between alternate form and polymorph self, the major one is this stipulation on Polymorph:
The assumed form can’t have more Hit Dice than your caster level (or the subject’s HD, whichever is lower)
Now, the Shape of Fire as a CR 26 creature has 35 hit dice, and some terrifying special abilities commensurate for a CR 26 threat. A phasm is a CR 7 creature with 15 hit dice. If you consider that the target should not have more hit dice / levels than your current form as an important attribute of a polymorph spell, then these effects are not similar at all. One allows you to change into any CR of target, the other allows you to change into a same or lower CR.
Assume Supernatural Ability then will not work, because you did not assume the Shape of Fire with polymorph or a similar ability. I think this is a quite convincing argument against this working.
But lets assume it works, your DM is up for shenanigans because they can get you into trouble. Then it is not clear how long you retain control over whatever lavawights you create because:
Lavawights are under the command of the shape of fire that created them and remain enslaved until its death.
When your polymorph spell ends afer a few minutes, you revert to your own form, and you are not a shape of fire any more. But you have not died. Maybe that means you still can command them, maybe not. If your DM decides that not, all you have is a costly way to create uncontrolled lavawights.
If the DM decides you can both create them and keep control of them if you revert, you instead have a way to create an unending army of obedient lavawights for the cost of two feats.
In case it's not clear already, changing form is among the top 5 most powerful game elements a player can marshal in Dungeons & Dragons 3.5. If you're considering for your PC in a typical campaign changing form to the degree this question describes, you should really run this plan by the DM before showing up to the table with it.
A level 15 wizard who can cast the 4th-level Sor/Wiz spell polymorph [trans] (Player's Handbook 263) can take the feat Assume Supernatural Ability (Savage Species 30–1) and pick for that feat the supernatural ability alternate form of a phasm (Monster Manual 208).
The prerequisite of the Assume Supernatural Ability feat says that the creature must have the "ability to assume a new form magically." The prerequisite does not say that the creature that takes the feat must be able to assume the form that it draws the supernatural ability from. (That, I assume, is the dude who takes the feat's problem.)
For example, an appropriately wise level 6 wizard can take at level 3 the Assume Supernatural Ability (phasm's alternate form) feat and take at level 6 the Assume Supernatural Ability (shape of fire's create spawn) feat—if, at level 3, the DM ruled that the 2nd-level Sor/Wiz spell alter self [trans] (Player's Handbook 197) were a "similar effect" to the obsolete 4th-level Sor/Wiz spell polymorph self [trans] (Player's Handbook (2000) 237). Savage Species is a bridge text that between the game's release version and its 3.5 revision, so Why a Revision? (Dungeon Master's Guide 4) totally applies.
Speaking of, a DM may rule that the first effect closest to a polymorph self spell is, in fact, the polymorph spell, making it so a wizard would, instead, be able—at the earliest—to take at level 9 that first Assume Supernatural Ability feat and at level 12 the second.
That same level 15 wizard can then cast the polymorph spell, assume the form of a phasm, and employ the phasm's alternate form ability to assume the form of a shape of fire (Epic Level Handbook 215). However, when that wizard in phasm form assumes the shape of fire form, the wizard renders the phasm form irrelevant:
One Effect Makes Another Irrelevant: Sometimes, one spell can render a later spell irrelevant. For example, if a wizard is using a shapechange spell to take the shape of an eagle, a polymorph spell could change her into a goldfish. The shapechange spell is not negated, however, and since the polymorph spell has no effect on the recipient’s special abilities, the wizard could use the shapechange effect to take any form the spell allows whenever she desires.… (PH 172)
In short, when layering forms, only the original (wizard) form and the last (shape of fire) form matter. Any other forms are suppressed. Thus, after the wizard changes form into the shape of fire, the phasm form is suppressed, and its alternate form ability gone. That alternate form ability—that the wizard no longer possesses—says, "A phasm can remain in its alternate form until it chooses to assume a new one or return to its natural form." And the wizard can't choose to use what he doesn't have. The DM makes a ruling:
There are, I'm sure, many other possible rulings, not the least of which may go something like this: "The cosmos's phasms realize simultaneously how dumb they were for not taking the Assume Supernatural Ability (shape or fire's create spawn) feat, undergo psychic reformation by allies who had been telling them for eons that they were underusing their abilities, and start conquering the multiverse."
If the wizard can assume the form of a shape of fire and possesses the Assume Supernatural Ability (shape of fire's create spawn) feat, then he can totally make that village of humanoid commoners into his army of lavawrights (EL 200). To be honest, that's a pretty solid fade to black (or red) for the end of a campaign, but, at the risk of minimizing what's actually a pretty clever late-levels trick, if the wizard's player's goal was an apocalyptic end to the campaign, he could've done that, like, way earlier. (Seriously, the wightpocalypse starts at level 1.)
Oddly, unlike most shapes of fire, when a typical human wizard 15 assumes the form of a shape of fire this way, he's still alive, keeping his type and subtypes rather than becoming an incorporeal undead. Because "[l]avawights are under the command of the shape of fire that created them and remain enslaved until its death" (215), most lavawrights are free-range. That wizard may be the only shape of fire who actually commands his own horde.
Note: Readers may also be interested in this question.
