666 (Trihexa) is a legendary being known as the Apocalyptic Beast and Emperor Beast of Apocalypse. At its full power, it is stated to be on par with Great Red and Ophis. Also, it is the Beast recorded in the Apocalypse.
666 has the appearance of a large beast with characteristics coming from different animals like a lion, a leopard, a bear, a dragon, etc. It has seven necks, seven heads, with ten horns, as well as seven long thick tails of different shapes. It also has four stout arms and two legs that are even thicker than its arms. Its main body is that of a primate leaning forward and is covered in black fur and what appears to be scales all over its body. Its size is well over several hundred meters, making it significantly larger than even Great Red.
The appearance of Trihexa's core is described by Issei and Ddraig in Volume 21 as a young boy made of light, whose only distinguishing feature is 10 horns upon its head. Soon after, this being sprouts six wings (bird, dragon, bat, and various other beasts).
Not much is known about its personality other than the fact it has only one nature and that is to destroy everything including the Underworld, Human World, Heaven, and all other mythological worlds. It's possible that it is completely mindless, only instinctively knowing to destroy everything. Its homicidal nature, combined with its immense power, made it immensely dangerous to the point that the God of the Bible had to risk his own life by using numerous forbidden seals to seal it away to prevent it from endangering the World.
Despite its supposedly savage nature, it seems to have an interest in Great Red and Ophis. This is evidenced by how it was attracted to Issei upon sensing their power in him. This could be because both Great Red and Ophis are beings on par with Trihexa.
At some point in the past, the God of the Bible confirmed the existence of 666 that was thought to be unknown and placed thousands of forbidden seals on it, sealing it at the "end of the world". In Volume 16 however, it was revealed that Rizevim and Qlippoth found 666 by using the Sephiroth Graal's ability to come into contact with the principle of life and decided to unseal him with the power of the Holy Relics.
Powers & Abilities
Immense Strength: Trihexa's powers were initially largely unknown due to being sealed for many years. Trihexa has power on par with Great Red and Ophis in its prime; a clash between both Trihexa and Great Red is feared because of the potential damage it could inflict to the world due to their strength, with Azazel believing that a clash between them, will result in the end of the world.  In fact, it is such a true monster that it could easily destroy the entire world on its own. Rizevim stated that it took thousands of forbidden level seals for the God of the Bible to seal it away. Upon gazing at it for the first time, Azazel notes that just its aura alone is enough to make a normal person go crazy. According to Shiva, only Ophis at full power and Great Red can destroy it as Trihexa's absolute overwhelming power made it impossible for Shiva, who is an immensely powerful God, to destroy it. In a matter of days after being unsealed, it managed to cause massive damage to multiple mythological factions, nearly completely wiping out Heaven, Grigori, and Asgard. Trihexa's strength is so immense that a coalition of the strongest beings from each mythology is required to defeat it, in a process that could take thousands of years.
Immense Durability: Trihexa has nigh-indestructible skin, being mostly unfazed from powerful attacks used by God-class beings. It was able to withstand attacks from Vali Lucifer's Balance Breaker without any damage, and is described to be so powerful that an immensely powerful God of Shiva's caliber cannot destroy it. Additionally, Issei Hyoudou's imperfect Apocalypse AnswerArms, which is said to be capable of destroying an entire planet, is incapable of fully destroying it with a single attack. Each of its individual bodies are able to withstand attacks from powerful Longinus users like Issei and Dulio. Coupled with its regeneration abilities. Trihexa's resistance is extremely high.
High Speed Regeneration: Aside from its natural durability Trihexa possesses high speed regeneration and is able to heal itself in an instant if any parts of its body were damaged, being able to regenerate even if turned to ashes or completely destroyed by the Power of Destruction. Due to its immense regeneration, Trihexa can easily survive immensely powerful attacks to the point the Balance Breakers of extremely powerful Longinus possessors such as Dulio, Tobio, Issei, and Vali could not inflict any permanent damage on Trihexa's body. According to Sirzechs, even with the combined force of the numerous Gods of various mythological factions, Azazel, Michael, and three of the current Satans including Sirzechs, a Super Devil himself, it will take an enormous amount of time to be able to defeat or even seal Trihexa due to its tremendous regeneration powers.
Plate Mail Production: With the help of the Khaos Brigade's modifications with the Sephiroth Graal, Trihexa can mass produce Boosted Gear Scale Mail armors. This ability was lost after Gasper and Valerie retrieved her Holy Grail cup.
Body-Splitting: Trihexa can split each of its heads into seven different bodies. Despite being much less powerful than its complete state, each of its individual bodies are more powerful than average God-class beings.
Core Expulsion: Trihexa has shown the ability to release its core. It demonstrated this in Volume 21 when one of its cores appeared before Issei due to Trihexa's curiosity when it sensed both Great Red's and Ophis' powers coming from him.
- The name "Trihexa" can be broken up into two parts: "Tri" which means "three", and "Hexa" which means "six". So "Trihexa" can be interpreted as "Three Sixes" which can refer to the name of 666, which has three sixes.
- Light Novel Volume 16, Life 4
- Light Novel Volume 16, Afterword
- Light Novel Volume 20, The Beast 666
- Light Novel Volume 21, Afterword
- Light Novel Volume 20, Deterrence
- Light Novel Volume 21, Khaos Disaster
- True Light Novel Volume 2, Life 3
- True Light Novel Volume 3, Life 4
- Light Novel Volume 21, Life 2
- Light Novel Volume 21, Last.DxD