Mythic Blood comes with a "power" of level up to 30, that can be invoked at will. ArM5 p.47 says "The effect should be designed as a Hermetic effect", so by the rules as written it seems it could be a Ritual effect. Am I missing something?
While I do not see anything outright banning a Ritual effect, there is text which specifically compares it to a formulaic spell for casting speed and concentration. That would seem to imply that it should be a formulaic effect, though of course YSMV.
If you did allow a ritual effect, is Vis required?
I would not allow a ritual effect as the free one that comes with Mythic Blood. I could see Mythic Blood as justification for the Ritual Power virtue, an additional major virtue, to be taken with the standard rules for that per RoP:M p47.
I would not let it be used for a ritual effect.
Additionally, you may choose one special magic feat which you can invoke at will and cancel at will, as often as you like. Invoking this feat takes as long as fast-casting a mastered formulaic spell (see page 87), and requires the same level of concentration.
The speed of it being used implies to me it wouldn't work as a ritual power. if you did allow a ritual power, I would definitely require vis for it.
That brings us again to the question if ritual spells can be fast casted through mastery.
Yes.. I kind of dislike that question... I can think of no good reason to allow it for story or game continuity. It's not explicitly denied, and it's marked down as happening on some random NPC (can't remember where).