Speaking through his robotic double, Calus tells you that Gahlran's so-called Crown of Sorrow had a very peculiar effect on the Leviathan—it briefly split the seams of reality, allowing access to an altered plane of reality that Calus believes you're quite familiar with.

Calus's men ventured in and found the frame of a cursed gun. Since then, the gun's incorporeal essence has tormented the Leviathan's upper decks and eluded Calus's grasp.

Perhaps you'll have better luck than the Cabal.