What defines one's home plane?

Usually, outsiders and elementals born only in certain plane and that is their home plane for sure. But, some creatures, including those usually seen in material plane, are establishing communities in planes which their race does not typically belong to. For example, many mortal Orcs and Goblinoids (not fiendish nor axiomatic version) are living in Acheron. And while usually Rakshasas are "native" outsider and their home plane is material plane, there are considerable numbers born and living in acheron, too.

Well, then, are they native to Acheron or still native to material plane? I am wondering if PCs can use Dismissal on Orcs, Goblinoids, blue drogons, rakshasas and such living on (or in) iron cubes of Acheron.

Shin Okada said:

Well, then, are they native to Acheron or still native to material plane? I am wondering if PCs can use Dismissal on Orcs, Goblinoids, blue drogons, rakshasas and such living on (or in) iron cubes of Acheron.

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You, as DM, must decide where they were born. It would be best to have this decided before the combat encounter begins, though a random roll is fine for non-significant foes.

Of note, make sure the spell used IS dismissal or Dispel Evil / Dispel Law. Banishment and Holy Word / Word of Chaos can only send creatures away if the caster is on his home turf.

http://www.d20srd.org/srd/spells/dismissal.htm