The mythical titan Prometheus - magical or faerie?
I mean he sided with Zeus during the Titanomachy, and gifted humanity with "fire".
Though Zeus chained him to a rock (maybe on the axis mundi) and had an eagle eat his liver every day. Until Herakles came along and strangled the eagle.
Hmm,... Liver and immortality go together. Is the eagle dead, or surviving with a broken neck?
Quest item for a Longevity potion?
All titans are magic realm.
I was wondering if Prometheus was misnamed as a 'Titan", or a faerie imposter has overwhelmed the original story? Or something.
But now I am really wondering what that liver infused bird might provide, magical Blood-wise.
YSMV of course but I would probably have both. The magic titan Prometheus was freed by Heracles and is now doing whatever a free titan does. But I would definitely also have a faerie Prometheus, either in a regio or in the Faerie Realm of stories, so that a PC could play the role of Heracles and free him. Surely a powerful source of vitality for the faerie, despite the necessary liver-ripping. Vis from the bird, as you mention, would definitely be an appropriate reward for completing the story.
For the magical titan, perhaps the magical chains that bound him can still be found, and provide Terram vis (or just a useful magic item for binding magical creatures - a Might suppressing effect?). Perhaps the vis only manifests if someone touched by magic is bound to a rock with those chains for a season - there's all sorts of story and roleplaying potential there
Now I need to go back and read my favourite book from childhood - Diana Wynne Jones "Homeward Bounders"
Why not Rego vis? They're famous for being fetters, not for their material.
Definitely an option!
Another one I quite like (depending on the gore level of your saga), would be Corpus. Chain someone there for a season and their liver contains Corpus vis!