To my memory what the Nullstone did was nullify the contract between summoner and Voidsent, thereby returning them to the Void. Used to keep unruly Voidsent from getting too uppity. Wouldn't be too useful in a foray into the Void, and given our contract with Zero's mutually beneficial and she doesn't seem like the backstabbing type, I don't see why it would be needed.

Quote Originally Posted by Zero-ELEC View Post
Doesn't the Nullstone actually dissipate the aether of the voidsent? Wasn't this what Diablos did with Scathach in the final Shadow of Mhach raid?
Diabolos devours Scathach to become Diabolos Hollow, like how Ciriatto devoured Farfarello (though for very different purposes).