I redesigned the trillith as a solo in the mold of Monster Vault dragons, giving her ways of shaking off negative conditions, for instance. I forgot to give her a way out of being immobilized, though, which was problematic.
I had Gallo act heroic, spending his time trying to drag other nobles to safety and putting himself in danger.
I kept track of nobles dying left and right, making the party worry about saving as many of them as possible.
When folks went insane, I let them randomly go after anyone nearby, which was the trillith in some cases.