during the events of Tales of Symphonia where Sheena is making the pacts with the Summon Spirits they all talk about already having a vow with the Hero, Mithos (with the exception to Volt)
I am up to breaking out of Welgaia and have learned that Mithos is
Lord Yggdrasil, who's real name is Mithos Yggdrasil
now given the above and how Sylvarant and Tethe'alla work i could assume that Mithos's vow is something along the lines of
keeping the world split in 2 and/or to protect Martel from the Great Seed
however when Sheena went to make a pact with Volt, Volt said that Mithos broke his vow and this lead Volt to lose faith in people and not want to make a pact again.
Now Sheen's vow has changed over the course of the game so they don't all have to have the same vow, however all the spirits but Volt have not nullified their pact with Mithos so one would suspect that their vow is vastly different to Volt's
So what was Mithos's vow(s) to the Summon Spirits? particularly the one with Volt which was broken