Blood Brothers (3.5e Spell)

''Drawing the knife across his palm to bring forth a slow trickle of blood, the wizard reaches out to the fallen hound archon before him, who bears a similar incision. His magic seems to pull the blood from each wound and towards the other, until the two are joined by a river of intertwined sanguineous liquid. As you watch, the elderly wizard’s wrinkles seem to smooth over as he is filled with the energies of the Outer Planes.''

The caster initiates a blood transfusion between the caster and the target, who receives no save if they are unwilling but helpless and requires none if they partake of their own accord. This switching of blood is combined with magic to allow the caster to replace their normal type and subtype with that of the spell’s target, gaining all benefits thereof. If the caster wishes, they may instead give the target their type (and any applicable subtypes) or combine both effects to swap types.

This spell does not allow you to turn into that particular creature; it only augments your form to the extent that you take on their type and otherwise remain your normal self.