At some point in time, he was diagnosed with a rare disease, but survived. For contrasting reasons (homicidal rage and regulating human overpopulation, respectively), Curien and Goldman masterminded incidents involving hostile biologically-engineered creatures in 1998 and 2000, which resulted in their deaths. Thornheart's level of involvement in these incidents is unknown.
Goldman staged a posthumous incident in 2003, scheming to revert self-destructing humans to their "natural state" in the opening of "Pandora's Box". Sometime before, during, or after these events, Thornheart resided in an office, chiding Goldman's "soft" intentions and revealing that there was more than one Pandora's Box.
The second Pandora's Box was destroyed by AMS agents G and Kate Green after a battle with the Magician, a creature originally made by Curien who was resurrected. Thornheart wandered into the Magician's chamber, remarking that while Pandora's Box is "closed", a being named the Wheel of Fate cannot be stopped.
On December 6, 2006, Thornheart unleashed creatures upon party guests at Scarecrow Manor. In secret laboratories, he developed creatures with the purpose of eradicating humanity in a plan known as "Noah's Ark". Although confronted by Kate Green and Ryan Taylor, Thornheart escaped and unleashed the Moon, a massive creature. He developed more creatures, claiming that the ordeal "has just begun", and that it is impossible to change a "predefined future."
By 2019, most of humanity was decimated following the incidents. AMS personnel and other survivors investigated the EFI Research Facility, inside which the Wheel of Fate -- a resurrected form of Curien -- was to destory humanity altogether. Although the Wheel of Fate was destroyed, Thornheart limped into his chamber and picked up a glowing canister (revealed earlier as the cure for Curien's son Daniel), remarking that it serves a "true purpose" before leaving.
The House of the Dead III
"It appears that he didn't understand its true purpose..."
The House of the Dead 4
"Goldman, you are soft. These wretched humans have no need of hope. Soon the true end shall begin. There is more than one Pandora's Box."
The House of the Dead 4: Special
"So, Pandora's Box has been closed... That is, if there is any "hope" left. The Wheel of Fate cannot be stopped."
House of the Dead: Scarlet Dawn
"There's more to come! Run until you die!"
"Curien, Goldman, it's time for humans to decide: Extinction or evolution?"
"Goldman took evolution to be biological adaptability, [sic] but I consider it to be truly about the human mind. Arrogance, greed, envy, rage, lust, veracity, laziness... All great expressions. We need to get rid of the dead wound. It's time to carry out Noah's Ark! Let's start a new evolution!"
"If human beings are going to die, just let them die. Finally...the new human evolution has begun!"
"Goldman and Curien were gone, but I was reborn from a fatal disease. I choose to be alive!"
"Can't you see it? The future looks plural, but in truth--there is just one! I can see it with my eyes!"
"It is the time...The time for new life to be born in a new evolution!"
"It's impossible to change a predefined future. Your ordeal has only just begun."
Before his identity was revealed in Scarlet Dawn, House of the Dead fans often nicknamed Thornheart the "Mystery Man".
His appearance varies throughout the games. In 4, he appears to be a middle-aged and normal-looking man, however in the sequel Scarlet Dawn (which takes place three years later), he is much older-looking, with pale skin, white hair and whitened eyes, which he looked like years before the game series began as evidenced in an old photograph. In III (which takes place 13 years after Scarlet Dawn) he appears to be slightly younger and back to normal-looking, although this was possibly due to the engine and the fact he is obscured and not to be fully seen. It is not yet explained why this is the case, although it is possible the condition he has in the storyline has something to do with it.
His laugh in III's ending is similar to Goldman's laugh from TheHouse of the Dead 2.
In a 2012 interview for The Website of the Dead, series director Takashi Oda stated that an arcade sequel would be based around the character.Scarlet Dawn, released six years later, would reveal Thornheart's identity and establish him as the main antagonist.
↑SEGA Interactive R&D#1. (13 September 2018). House of the Dead: Scarlet Dawn. Sega. Level/area: Laboratory. Kate Green: [Explaining a photo of Dr. Curien, Caleb Goldman, and Thornheart to Ryan Taylor]In 1880...the organization their ancestors made was...the beginning of everything.
↑SEGA Interactive R&D#1. (13 September 2018). House of the Dead: Scarlet Dawn. Sega. Level/area: Manor Depths. Thornheart: Goldman and Curien were gone, but I was born of a fatal disease. I choose to be alive!
↑Sega Wow Inc. (30 December 2005). The House of the Dead 4. Arcade and Playstation 3. Sega. Level/area: Thornheart ending. Thornheart: Goldman, you are soft. These wretched humans have no need of hope. Soon the true end shall begin...There is more than one Pandora's Box.
↑Sega Wow Inc. (30 December 2005). The House of the Dead 4: Special Arcade and Playstation 3. Sega. Level/area: Thornheart ending. Thornheart: So, Pandora's Box has been closed...That is, if there is any hope left. The Wheel of Fate cannot be stopped.
↑Sega Wow Inc. (17 September 2013). The House of the Dead III. Arcade, Xbox, PC, and Playstation 3. Sega. Level/area: Thornheart ending. Thornheart: It appears that he didn't understand its true purpose...[laughs]