If it also fails this Fortitude save, the subject permanently assumes the form of the chosen animal or humanoid, as per polymorph. This is a polymorph effect.
Only the part that polymorphs the target.
So now we have Illusion (shadow) spells that substitute for polymorph spells.
Some Developer has a bad habit of creating spells that poach other Schools specializations and giving them to classes that have no reason to have them: Alchemist, Bard, Druid, Skald.
Really should have just said it works as Baleful Polymorph because that's basically what it is.
There is an Illusion (Shadow) part to it. It is similar to the Shadow magic spells. That kind of spells had a raison d'être in 1st ed. AD&D, when the illusionist was a different class from the wizard, but now it is simply poaching other schools spells.
Checking the Archives of Nethys I see 15 shadow spells that duplicate what one or more spells of other schools do.
Nice if your game is set at Nidal or on the plane of shadows, annoying when every player want what should be rare specialist spells as they have seen them on the Archives.