I do recognize that things have been going a bit too well for our heroes. Let's take a look at what our villains are doing, shall we?
Shambhala was generally fairly orderly. Thales kept an iron grip on his city, and any trace of rebellion was met with extreme prejudice. If any rebellion happened, it was where the shadows were so thick that even Those Who Slither In The Dark couldn't see.
But usually, rebels in Shambhala lacked the power to stand up to Thales's forces. That was far from the case here, as evidenced by the guards erupting into flame.
"I found you," Corvis said in a sing-song tone as he confronted Thales.
"Corvis," Thales spat.
"When was the last time we saw each other, the War of Heroes?" Corvis continued. "Why don't we compare what we've learned since?"
"If you want to die, who am I to stop you?" Thales said. With that, he cast a Thoron blast at the renegade, only for Corvis to effortlessly avoid the attack. He then tried to retaliate with a powerful Arcfire spell, but Thales simply batted the spell away.
Thales began using Miasma, spraying a poisonous cloud into the air. "You think that scares me?" Corvis asked dismissively, throwing more flames at the Agarthan king.
Thales simply avoided the flames, either by dodging or deflecting them. However, as he attempted to approach Corvis, he found himself having to quickly avoid a jet of flame from the floor.
"Did you not realize I could do that?" Corvis asked, as Thales had to quickly avoid more of Corvis's traps. "You won't find it so easy to attack me. However, I can go after you with impunity!" He continued throwing fire at Thales.
The battle continued for some time, with Thales unable to get any strong attacks in, while constantly having to avoid Corvis's fire.
"Your impatience will soon be your undoing," Thales told Corvis. "Do you not feel it getting harder to breathe? Your fire attacks have done an excellent job of consuming the air. I, of course, am unaffected by my Miasma spell, but you will be unable to last for more than one more minute."
"One minute is all I need," Corvis retorted. "Many of my flames weren't aimed at you."
Thales suddenly realized where Corvis had actually aimed his attacks throughout the fight: the structural supports within the walls and pillars of the throne room. As he realized the renegade's trick, the ceiling began to collapse above Thales.
Thales reached out his hands to create shield spells, one above him and one facing Corvis. Corvis cast one last explosion spell, but not at Thales.
His true target was a piece of the falling ceiling behind Thales. The explosion tore the bit of rubble apart. Bits of it flew in every direction. Including several pieces that hit Thales in the back, with the force of arrows straight from a bow.
But there was a consequence. Between the power Corvis had been using so liberally, and the smoke and Miasma in the air, his movements were growing slow. And when Thales's shield had held against the rubble that had been meant to crush him, Corvis was unable to dodge the dark tendrils of Thales's Nosferatu spell.
Corvis felt his energy being drained to heal his foe, and quickly lost consciousness.
Some time afterwards, Corvis regained consciousness, which was strange, because by all logic he should be dead.
"Did you really think I would kill you just to heal wounds that could easily be fixed by healers?" Thales asked. "You hold too much value to be wasted so lightly. Or at least, what you have within you does."
"The Crest of Periander," Corvis realized. "Your Agarthans have Crest Stone hearts of their own, through eugenics and dark magic, no doubt. But they don't compare to the Crest Stone of a Nabatean, do they? You intend to use me as a sacrifice."
"You always were clever," Thales affirmed.
"It will do you no good," Corvis told him. "Even if the Fell Star gets no information out of Odesse, I sent a letter to Emperor Edelgard with the location of this place before I came after you. You cannot repulse the entire Imperial Army."
"Hmm. It seems the Flame Emperor has outlived her usefulness then," Thales mused. "It matters little, however. In a few seconds, the greatest weapon of the Agarthans will be reborn!"
As the ritual began, and dark magics tore Corvis apart, he cackled insanely. "He who deals in death and destruction must not shy away when they come for him! I'LL BE WAITING FOR YOU, THALES!"
Thales paid his last words no mind as the ritual reduced him to nothing. Once every last trace of Corvis was gone, the coffin that held the Agarthans' greatest weapon opened.
Out emerged Nemesis, the Butcher King. The greatest weapon the Agarthans had ever wielded.
"I live once more!" Nemesis declared.
"Nemesis," Thales said. "Now that you have returned, our supremacy is ensured!"
Nemesis walked over to ten other coffins. "Rise, my Elites!" As he issued the command, the coffins opened, and the Ten Elites rose from the coffins.
Thales turned to his subordinates. "Pittacus, Bias, begin preparing as many beasts as possible for war, Demonic and of other sorts. When we meet the humans in battle, I want them to think the legions of Hell are upon them. Myson, contact Cornelia. Begin the Contingency." The Contingency was the plan for if the Flame Emperor plan fell through. Thales, Cornelia, and several other agents would cause civil wars within the three nations of the continent, weakening them enough to allow the Agarthans to begin their conquest. "Nemesis, I have a request for you. The asset we used to cause the war in Fódlan is known as Edelgard von Hresvelg, ruler of the Adestrian Empire. She served her purpose admirably, but her usefulness has come to an end. Kill her."
"Wilhelm's get," Nemesis spat. "Killing that fool was immensely satisfying. Seiros may have been a fierce opponent, but even if I consistently failed to injure her, I proved many times that I could hurt her." He grinned sadistically. "I will take pleasure in ending his line."
"One more thing," Thales told him. "The Fell Star has manifested a new incarnation. He now wields the Sword of the Creator."
"That sword is mine!" Nemesis snarled. "I will tear it from his hands!"
"Indeed. But until you do so, you will require a new sword," Thales said as two Agarthan mages presented him with an enormous black greatsword, one no ordinary human could wield because of its size. "Hate Eternal is its name. It is our weaponsmiths' finest work since the Sword of the Creator itself, imbued with dark power that no other blade can match."
"It is a fine blade," Nemesis agreed. "I will wield it gladly. And once I cut the Fell Star down, I shall wield this blade in my left hand and the Sword of the Creator in my right, and all who challenge me shall die!"