Subtype Transformation

[Ruethgar]

Is there anything short of polymorph that could add the Tanar’ri subtype?

[Karnith]

The Savage Species Ritual of Association can probably do it, but you won't get any benefit of the subtype beyond using items as a member of that subtype. You also get a +5 bonus on Disguise checks to pass yourself off as a creature of said subtype.

[Ruethgar]

Pricey for the level I was looking for but doable. Thank you, now I can get retribution aura without becoming a walking bomb.

Edit: On second thought, would being a walking bomb be that bad? Would need a good fortitude save, but you could eek out some decent damage that way. Can you willfully fail a save?

[Splendor]

Fiendform spell (PGF 102) can turn you into a dretch (which is a Tanar'ri).

Fiend-Blooded prestige class in the Heroes of Horror turns you into a half-fiend, but no sub-type.

[Ruethgar]

Fiendform doesn't add the subtype. Whithout it, whenever I take damage I also take 50% more on a successful fortitude save(including from the 50% extra damage, +25%, +12%, +6%, +3%, +1%). Can't be negated, reduced or blocked by damage immunity, DR, SR, or resistance, only by Mettle and Subtype.

I can become a demon just with alter self and an outsider race(otherworldly deep Imaskari).