Is it possible to lose a Mystery Virtue?
Perhaps a bad Twilight episode leave the maga having forgotten how, or unable to get into the right mindset, to use a Mystery Virtues?
Would some Mystery Virtues be unable to be lost? - Faerie Magic, Heartbeat, the Enigma, Verditus magic?
I don't think there is anything restricting what virtues can be lost outside of troupe consensus. Some might need a narrative explanation about how they are lost, and losing them could be a fundamental change(you're blood is permanently purified of any faerie influence) or a mechanical one(you still have faerie blood in the narrative, the change is you have lost your connection to it's power and no longer have the virtue). Now of course if a player made a character around a particular virtue snatching it away from them without their buy in would be a bad idea no matter what the rules or setting technically allow.
There's nothing that talks about losing virtues at all (due to a Twilight episode), unless there's something in another book that I'm not remembering that didn't make its way into the DE. You can gain a virtue, and you can gain flaws, but you can't (as the rules are written) lose a virtue.
I would be very hesitant to take away a mystery virtue in any event, as gaining them requires a lot of time and sacrifice, and doubly so to touch virtues that are integral to the Magi's house.
Check TMRE p.14: "Gaining a Major Flaw or losing a Major Virtue is a Major Ordeal." "Gaining a Minor Flaw or losing a Minor Virtue is a Minor Ordeal".
Now that I have the book in front of me, I see I did misremember. Under the "Lost Knowledge" part of a bad Twilight, it is only affecting a very limited set of abilities.
But this limited set includes Enigmatic Wisdom. What would happen if a maga managed to lose every single experience point in the supernatural ability associated with the Outer Mystery of Enigmatic Wisdom? Have a score of zero - mechanically a character can't learn an ability-linked Virtue unless they learn the associated ability to score 1 in a single Season. So if you have zero XP in the Virtue linked ability, do you have the Virtue (in this case Enigmatic Wisdom)?
I only mentioned a bad Twilight at the beginning because it was the only example I could recall that might possibly remove a Mystery Virtue. Are there other methods, such as @OneShot 's suggestion?
I am technically looking for bizarre motivations for an NPC.
I think you can reduce the experience in an ability such as Enigmatic Wisdom without necessarily losing the associated virtue, which allows learning it back.
Can you have an Enigmatic Wisdom score < 1 if you have The Enigma virtue? The virtue says "You have a score of 1 in Enigmatic Wisdom." If you haven't lost the virtue it could be argued that you've still got a score of 1.
If it's for an npc it should just be whatever fits the story best. As much as the game tries to give mechanics to magic it deliberately leaves mystery in many aspects such as twilight and realms. Having anything happen because of warping or wizard's twilight is pretty much always going to be narratively fair game.
Enigmatic Wisdom and Heartbeast are Arcane Abilities that you need an Initiation to learn, not Supernatural Abilities. Though even if it somehow undid the initiation, the process of learning Enigmatic Wisdom is just 'spend a year at the Cave of Twisting Shadows learning from the Criamon'.
You can lose a virtue via a pilgramage. The power of God is omnipotent and likely doesn't care if the virtue is a mystery virtue or not, at least in regards of capability.
Presumably it should also be possible to lose virtues that are representative of arcane, hard to grasp mysteries via perdo mentem. That said I'd be hesitant to let that stick as its indicated all initiations change the Essential Nature.
The nature of the Initiation pro-
cess makes the changes a true part of
the magus, affecting the magus’s essential
nature. Thus magic cannot undo Initiation
effects. Even if a maga subsequently leaves
a Mystery Cult, she does not lose the
Virtues and Flaws she accrued.
Pg 11 of the mysteries
Hence removing a mystery virtue by anything less than a change of the Essential Nature seems like it should be temporary, or at least reservable. In the example above, I'd say Creo Mentem could restore the memories since there's always at least a fragment from the Essential Nature of the initiate.
That said, a mystery virtue sounds like a suitable sacrifice for another initiation.
So, there's
Broken Vessel
Minor, Supernatural
The character is constantly aware that her supernatural powers may only be temporary. Failures using those abilities represent the ebbing away of the power that sustains them. When a zero is rolled on any Stress Roll (not Simple Roll) for any of her Supernatural Abilities, Powers, or in casting spells, she loses 5 experience points from that Ability etc etc etc.
which to me implies that at the very least some mystery virtues can be lost, or at least, the power can be. That flaw doesn't say anything about what happens when you hit zero though.
I might suggest that instead of losing the virtue, they gain a (minor/major as relevant) variant of the "Suppressed Gift" flaw, applying to the supernatural ability(s) instead of The Gift, if you think there's any chance that character could get it back. It's less punishing for the player character at the very least.
I do think that reversing an initiation in some way would be a fun way to ritualistically sever someone's access. Eg, if they sacrificed an eye in order to gain Second Sight (as in the Folk Witch initiation), tying them down and Creo Corpus restoring their full vision. It would then have the fun mechanic of "This antagonist has to discover the nature of the initiation," so the mystagogues get a target on their backs.
Unfortunately that just fails. Their sacrificed eye is part of their Essential Nature and thus cannot be restored with ordinary creo corpus magic as I quoted above- perhaps divine holy methods and powers could do it.