This blast only works on living creatures, and has no special effect until a victim fails a Death Test due to damage from Polymorph. The target then permanently changes into something else, selected when the effect is bought. This can be a statue of stone or lead, a pig, slug or whatever. A named target keeps his own mind, an unnamed target is changed in mind both and form.
The change must either be into another viable living creature, or retain the subject's basic form. A toad or basalt statue is OK, a pile of sand or a fish on land is not. You select what you change people into when the effect is first bought. Polymorph is related to Polymorpism, if you have that Sorcery schtick you can elect to transform your target into any form on a whim, as long as you obey the rules above.
See the Remove Curse effect under Divination for how to dispel permanent magic.
Common Metablasts
Area-Ball, Armor Piercing, Focus (depending on source).