Did mats or later content expose how Vritra's invincibility works? Her event implied it was like a sum of ''Damage Reductions'' based on the amount of conditionals that were achieved (day and night, warmth and cold object etc.) as George noticed Karna and Beowulf landed a bit of damage unlike the heroes hacking at her with tools so they got one less conditional. But I remember them mentioning she needed the grail for that so I don't know if she needed the Grail because it's some sort of Authority that got reduced for being a servant or because it has a super high mana cost.