To explain the underlying game mechanics, this would be one of strongest single attacks possible in the game, short of NP400% or NP500%. Their NP gets bonus damage the lower their HP is The base is itself 1200% normal attack damage to a single target, with another 1200% as the maximum bonus damage from low HP. 1% of their max HP is as strong as it can be. So that's 2400% normal attack damage.
However, that's at level 1. This is at level 5, with their NP bar charged to 300%, which is 1800% base with 1600% bonus. I believe these are additive rather than multiplicative, so the NP would deal 3400% normal attack damage. Let us assume this is also after their Interlude quest that strengthens their NP, 3800% instead.
However, having a max-ascended Black Grail grants a +80% damage multiplier to that result, with the drawback of losing 500 HP per turn...which works perfectly with this NP, needless to say.
The Black Grail can also grant a massive amount of attack stat points if it's been fully leveled up (this is separate from max ascension).
Whatever's on the other end of this attack, it's not going to be there afterwards.