Luca's suspicions weren't baseless.
The Nightmare Cult worshiped the Abyssal Demon Lord, who was none other than the Lord of Nightmares, Jormungandr.
If Luca's memory from his past life served him right, the second demon lord to descend into the real world was also Jormungandr.
The King of Rage, Hydra, had been the first.
That's why Hydra was known as the First Demon Lord.
Luca couldn't believe that the Nightmare Cult's presence in Riverfall City, along with their elaborate scheme to trap all the city's strongest warriors, was solely to help Hydra descend.
Their plans were undoubtedly grand, and helping Hydra descend might only be a secondary goal.
Their primary objective was likely to take advantage of the chaos and bring forth their Lord of Nightmares.
There was no other reason they would go to such lengths.
Why else would they drag the entire population of Riverfall City into a nightmare?