Polymorph/Metamorphosis, Fiendform, and the Outsider type

[Heliomance]

So, using Polymorph, Metamorphosis, or similar effects, you can only turn into an Outsider if you're already an Outsider.

The Fiendform spell (among others) temporarily grants you the Outsider type, letting you Polymorph into a different Outsider.

What happens if you do this, Polymorph/Metamorphosis into some other outsider (I'm planning on using a a Psychoactive Skin of Proteus), and then the duration of the Fiendform expires? Does the Metamorphosis break? Can you keep on being an Outsider?

[jiriku]

Sort of? The duration wouldn't technically break, but you're now an illegal target for the spell. You could probably make an argument that the spell "only checks once", but many DMs would find that argument unpersuasive.

[Heliomance]

Sort of? The duration wouldn't technically break, but you're now an illegal target for the spell. You could probably make an argument that the spell "only checks once", but many DMs would find that argument unpersuasive.

But would you be? Because the Metamorphosis would override the Fiendform, so the Metamorphosis might then self-qualify.

[Renen]

Well, the spell says "you assume a form... bla bla... not outsider unless you are one... bla bla..."
I'd say it only checks the moment that you are assuming form.
You never stop being a valid target, as the target is just "you", but you cant pick outsider as a form unless you are one. If you are one at the time of picking, you are good.

[Duke of Urrel]

For whatever it's worth, according to page 87 of the D&D Frequently Asked Questions (http://archive.wizards.com/default.asp?x=dnd/er/20030221a) (30th of June, 2008):


A spell only checks to see if you are a legal target when it is cast. If you become an illegal target later (such as via the Polymorph spell), the spell remains in effect.

[A_S]

But would you be? Because the Metamorphosis would override the Fiendform, so the Metamorphosis might then self-qualify.
I think it does self-qualify. Polymorph/Metamorphosis explicitly change your type. You're a valid target when you cast the spell, because of Fiendform. So the Polymorph effect is applied. Then you're an outsider for two reasons: Fiendform and Polymorph. When Fiendform expires, you're still an outsider, because you have Polymorph in effect. There's no time at which you cease to be an outsider (causing the Polymorph effect to end) until/unless the Polymorph effect ends.

That reasoning, plus the FAQ ruling, plus what Renen said about never becoming an invalid spell target...I'm convinced.