Ultimately, they breach the final chamber, and encounter the Wyvern Chiropteran- Now armed with the Omega Regalia. With the might of Kamen Rider Orga boosting his powers, he's nearly invincible. The fight is merciless, but terribly uneven; He has Dio's firepower, Longinus's strength and resilience, and Kerebos's speed, combined with his Gigersque shapeshifting abilities- Like firing flechettes of bone, claws that bleed acid, *wings*, and breath that turns organic matter to ash.
Mortally wounded, Dio tackles Orga from behind- And with the last of his strength, pins him, just long enough for Longinus and Kerebos to destroy the Regalia with a combined, and old-fashioned, Rider Kick. Now reduced to his Chiropteran form, he's still deadly- But working in concert, the two Riders nickel-and-dime him to death. Longinus returns the favor by ripping off both of his wings, maiming him- Before the Chiropteran is killed, by being guillotined by a closing bulkhead.
However, immediately after the BBEG's defeat- Longinus turns on Kerebos, striking him from behind with the Nail. He states that Rui Lee has to die, to prevent a Chiropteran outbreak from occuring again- Kerebos, of course, has other ideas. On their last legs, the two men duel- With one fighting for love, the other for the survival of the species. Badly wounded by Kerebos, Longinus damages Kerebos's gear- Putting them both on equal footing, unable to transform.
Then it comes down to bare hands and the strength to strangle. Kerebos partially blinds Longinus, by gouging out his left eye with his fingers- Longinus used Improved Grab to Pin Kerebos, and throttles him to death with his mechanical right hand. The sole survivor, Longinus retrieves a pistol, and limps away from the burning ruins, as the overloaded reactor ignites the underground fortress.