A synthesist summons the essence of a powerful outsider to meld with his own being. The synthesist wears the eidolon like translucent, living armor. The eidolon mimics all of the synthesist’s movements, and the synthesist perceives through the eidolon’s senses and speaks through its voice, as the two are now one creature. The synthesist directs all of the eidolon’s actions while fused, perceives through its senses, and speaks through its voice, as the two are now one creature.
While fused with his eidolon, the synthesist uses the eidolon’s Strength, Dexterity, and Constitution, but retains his own Intelligence, Wisdom, and Charisma. The synthesist gains the eidolon’s hit points as temporary hit points. When these hit points reach 0, the eidolon is killed and sent back to its home plane. The synthesist uses the eidolon’s base attack bonus, and gains the eidolon’s armor and natural armor bonuses and modifiers to ability scores. The synthesist also gains access to the eidolon’s special abilities and the eidolon’s evolutions. The synthesist is still limited to the eidolon’s maximum number of natural attacks. The eidolon has no skills or feats of its own. The eidolon must be at least the same size as the synthesist. The eidolon must have limbs for the synthesist to cast spells with somatic components. The eidolon’s temporary hit points can be restored with the rejuvenate eidolon spell.
While fused, the synthesist loses the benefits of his armor. He counts both as his original type and as an outsider for any effect related to type, whichever is worse for the synthesist. Spells such as banishment or dismissal work normally on the eidolon, but the synthesist is unaffected. Neither the synthesist nor his eidolon can be targeted separately, as they are fused into one creature. The synthesist and eidolon cannot take separate actions. While fused with his eidolon, the synthesist can use all of his own abilities and gear, except for his armor. In all other cases, this ability functions as the summoner’s normal eidolon ability (for example, the synthesist cannot use his summon monster ability while the eidolon is present). This ability replaces the class’s eidolon ability, bond senses, and life bond.
So, regarding the bolded sentence, how does one determine whose HD to use for the purposes of determining the effects of some evolutions (e.g. Frightful Presence)? On the one hand they are one creature now, the summoner is accessing the ability, and the above rules say nothing about the summoner using the eidolon's HD. On the other hand all evolutions reference the eidolon's HD. Is there something more that would settle this that I'm missing or is it a matter of how your GM interprets the above rules? The intent can be argued either way, so I'm not interested in what anyone thinks the intent was. I'm just wondering whether or not there is some other rules interaction that settles it from a RAW standpoint. Regardless it's a thing I'd run by the GM, but knowing any additional rules to point toward is always nice.