Who is Gareth?
by Hervé MusseauThis could be any immortal you wish to create, since we don't know what lies beyond the Vortex, nor who went there. You can then use the scenarios described for various immortals in the other posts, as fits the personality of your returned-from-Vortex-Immortal.
However, two Immortals are worth a special mention here: Verthandi and Sinbad.
a. Gareth = Verthandi
Verthandi is a very old immortal whose current whereabouts are unknown. He's from Time and has about the same portfolio that Khoronus, and reportedly was a Hierarch.
If IYC you make the assumption that Verthandi = Khoronus, you have an interesting Immortal who sponsored himself at least once and was the reigning hierarch of Time twice. A good candidate for Old One status, no? (Which self is the oldest one does not really matter, i.e. which one dropped immortality and which one searched for Old One-dom, and when, are irrelevant at this point thanks to the time-travelling usual to Time Immortals.) Whether he is an Old One or just an Immortal who went through the Vortex you can decide IYC.
If an Old One, he may be the one who tampered with the NoS to stop WotI and save the Old Ones' experiment (of which he is part, but Immortals of Time are used to cope with such paradoxes), and has become Gareth to remind mortals and Immortals alike the lessons of history. This mean you can combine the possibilities described in post #6 (Old One) and post #5 (new initiate) where Khoronus is discussed.
If just an Immortal who managed to venture beyond the Vortex, Khoronus/Verthandi is one of the most powerful and knowledgeable (and wise) Immortal ever. It may be him who stepped in at the end of WotI and posed as an Old One, using his tremendous power to force Rad and Ixion to bow before him and draw Rad into the NoS. Probably the knowledge he discovered make him more powerful than other Immortals and undetectable by the Council that watch for Immortal interference on the Prime. He decided to create the alternate identity of Gareth to both remind mortals and Immortals the lessons of history, and give them the possibility to become enlightened as he now is after his voyage beyond the Vortex by showing them the correct path (but not enforcing it, as it's self enlightenment, not brainwashing).
b. Gareth = Sinbad. Sinbad is a traveller. What he really is, is unknown; the Immortals hypothesise he may be an Old One, or one of their envoys.
If an Old One, what has been said above for Khoronus, or in post #6, could be applied with Sinbad.
If an agent of the Old Ones, then it's basically the same, except that he's not as powerful (but at that level it does not really matter, does it?)
We can consider a third possibility
: that he is simply a very old Immortal who managed to go beyond
the Vortex and returned (as said above for Verthandi). That is why
the other Immortals believe him to be an Old One or their agent,
while he in fact is just an Immortal who managed to do what they're
trying to. He decided to set up the alternate identity of Gareth to
help other discover the way to enlightenment as he did. However, to
achieve that you must open your mind. That is why Sinbad encourages
people to leave their homes and travel (as Gareth, he encourages
his followers to leave their homes and travel the world to spread
his words): it is a way to opening yourself to what you don't know,
going beyond your fears or your comfort, and discover what's
beyond. He also encourages "mental travel", i.e. fighting one's own
preconceived ideas (like the teachings of Immortals, for the
mortals; or the idea that mortals are but pawns, for Immortals) and
that they should focus on enlightenment (this is embodied by the
meditation and fighting style of his fighting monks/mystics; you
may also wish to introduce psionics here).