"Right! Vengancier is knocking on the door because someone here is wielding rift, and he himself doesn't even know who. Each of you, tell me if as far as you know you are the one who wields rift, my truth dagger will do the rest"

"No."
"No."
"No."
"No."
"No."
"Maybe."
"No."
"No."
...
"No."

... Eudial was the one saying maybe... the dagger isn't detecting any lies... they all genuinely think they are riftless... except for Eudial!

"SLEAZY LOOKING DEMON! You shapeshifting demon you. I will allow you to walk out of this castle, but only if you transform into Eudial... if you can't impersonate the voice let my singing lessons smoothen your throat for the benefit of your impersonation. Walk out of the castle... and admit to the ones waiting for you there that you wield rift!"