Is there any supernatural ability or Virtue that allows the summoning of magical creatures? I had only seen Muspelli's Summoning extended ability as a way to summon beasts with Might, but that's for animals, not for example magic humans. And Summoning works only with incorporeal beings on its Goetic version and with elementals on its Elementalist version...
if there is nothing I was wondering about creating an ad hoc new virtue allowing the summoning of magical creatures, but I feel that allowing it to work with material beings should be such an improvement to a virtue (or virtues) that are already Major.
So is there already a way to do it? And if there isn't, do you see any way to make it work?
There is HoH:S p.105 Summon Animals, a Major Supernatural Virtue allowing to summon mundane animals. This Virtue is present among Ex Miscellanea Beast Masters (HoH:S p.108), hence known to some Hermetic magi.
Extending this in a saga-specific way to magical creatures requires a means to give it high Penetration, and likely another Major Supernatural Virtue to extend its targets. While mundane animals are generic enough, magical creatures are often unique creatures, which a summoner needs to somehow specify.
As an alternative approach, a brotherhood of ReCo/An magi might have over centuries fixed and archived Arcane Connections to long-lived magical creatures, which they lend deserving brothers wishing to summon them. Such a brotherhood needs no new Virtue, but might be angered if a magical creature they hold an Arcane Connection to gets destroyed.
Theoretically you can summon anything you have an Arcane Connection to, or that you can directly perceive. Summon Animals (Muspelli use this, it's just Trolldomur allows them to boost it to summon supernatural animals too) breaks the limit of arcane connection in an interesting way, as noted in the description in HoH:S. Summoning virtues, like the Infernal, Faerie and Elementalist versions, and Sihr, all allow you methods of summoning quite specific creatures.
Any virtue you create should hopefully be explicit in what it allows you to do that Hermetic magic or existing virtues don't.