Synthesists with special attack evolutions

[thepuregamer]

So there have for a long while been some wholly ignored evolutions. For example frightful presence. On an eidolon whose number of hit die are low, this ability just doesn't work. But how for example does frightful presence work on a synthesist? The synthesist gets the evolution, so would it work off of his HD? If so, this could atleast be a useful ability as opposed to completely worthless.

[Mojorat]

my guess is it would still use the eidolons hd. but would use the synthesis charisma modifier.

[thepuregamer]

what about it the summoner picks it up through his greater aspect ability?