Mobius

"Universe after universe has fallen before my power. World after world consumed. With every death, I steal my mirror’s strength and mine grows ever stronger. Who am I? I am the Anti-Monitor. Come, Harbinger. There is work to be done."

- Anti-Monitor

Mobius, better known as the Anti-Monitor, is a being of supreme power and malevolence that embarked on a mission to eradicate the multiverse by whatever means necessary, opposed by his counterpart, Mar Novu, in which he was ultimately successful.

Creation
At some point, Mar Novu allowed for Mobius to live, a decision the former would come to immensely regret.

Imprisoned
Seeking to destroy the multiverse with waves of antimatter, the Anti-Monitor was sealed behind a door in a chamber where he built an anti-matter cannon to execute his scheme. When Barry Allen of Earth-90 charged towards Novu and was repelled, he found himself within the reach of the Anti-Monitor, who whisked him into his prison and forced him to run on a treadmill to power the weapon. During a test run of the antimatter cannon, the entire universe of Earth-2 was eradicated.

Messaging Nash Wells
On December 9, Harrison Nash Wells, mistaking the Anti-Monitor for Mar Novu, stood outside his door and taunted him, saying how he didn't believe in false gods. He argued with Nash to no end, when suddenly Nash was attacked by one of Ramsey Rosso's blood brothers. After a while, Ramsey and his army were stopped thanks to Allegra Garcia, but Nash believed it was "the Monitor" who saved him. At 11:58 pm, just as Nash was about to open the portal to what he believed would be the Monitor's realm, he noted on how he wished to know the truth and offered to reveal to him what he wanted to know in exchange for his service. He told him to stop his attempts to kill him and begin a new life. When Nash asked the Anti-Monitor to show himself, he said that he already has "the knowledge" (probably to solve a code). After Nash solved the code in the cave, the Anti-Monitor opened the rock and pulled him inside it in a flash of light before slamming down the rock wall again.

Released
Due to Nash opening the door, the Anti-Monitor was released and the Crisis of 2019 began. He began destroying numerous universes across the multiverse.

Corrupting Harbinger
Due to more worlds being destroyed, the Anti-Monitor soon communicated with Harbinger through unknown means and, despite her pleas to stop, forcibly summoned her to his lair. Once she arrived, he introduced himself. After revealing that worlds were falling under his power and every death allowed for him to steal more power from his mirror counterpart, he demanded Harbinger's cooperation and proclaimed that there was work to be done.

His successful endgame
The Anti-Monitor possessed Harbinger and after all except Earth-1 was destroyed and all the multiverse's refugees were brought to last remaining universe, he teleported to the Waverider and tried to fool the multiverse's heroes. However, they saw through his guise and futility tried to kill him. He overpowered the heroes and had his Antimatter wave destroy Earth-1 and the multiverse's survivors as well as murder Mar Novu as the heroes helplessly watch. As the wave was heading towards the ship, Pariah teleported the multiverse's last line of defense to a place where the Anti-Monitor could never find them, which angers him. Before they died, the heroes with their last breath warned him that this isn't over and those that Pariah saved will "fight to their last breath" just before the Antimatter wave destroyed them, completing his victory, despite that he failed to kill the Paragons, who are the only ones that can defeat him.

Personality
The Anti-Monitor had proven to be an incredibly evil being, wanting to destroy the entire multiverse, killing off an infinite amount of innocent lives without any guilt or remorse. He seems to enjoy taking credit for destroying universes and consuming worlds. He is also extremely intelligent and manipulative, able to convince the extremely intelligent Harrison Nash Wells into freeing him. Ultimately, his intellect prevailed as he succeeded in his mission.

He can also be described as a megalomaniac with a god-complex since he wants to replace the multiverse with his own realm to be absolute ruler.

Powers

 * Cosmic entity physiology: Mobius is stated to be far more powerful than Mar Novu himself, a remarkably powerful cosmic entity in his own right, making him one of the most powerful beings in the entire multiverse.
 * : When inside his prison, Mobius was able to communicate with Harrison Nash Wells when he was standing in the rocky wall that held the portal to his prison.
 * : Mobius was able to effortlessly pull Harrison Nash Wells inside the portal to his realm without even being physically present. He also used it when he's trying to communicate Harbinger; telling her that Lex is about to get the Book of Destiny, and he also tell her to go to him.
 * : Mobius took possession of Harbinger and used her body to kill Mar Novu and absorb his essence into her body.
 * : Mobius is able to grant powers to anyone he wishes, as shown when he turned Harrison Nash Wells into Pariah; thereby giving Nash the power to teleport and travel between dimensions.
 * : Through Harbinger, Mobius was able to induce a powerful blue energy blast to fight against Mar Novu, eventually overpowering him after a brief struggle.
 * : Mobius was able to absorb the essence of his counterpart, Mar Novu, killing him in the process.
 * : Mobius induced an antimatter wave that destroyed Earth-1 even when the Antimatter Cannon was already destroyed.
 * : Mobius was able to generate seismic activity while arguing with Harrison Nash Wells.
 * : Mobius has lived since the dawn of time.

Abilities

 * Genius-level intellect/Master tactician: Mobius is a highly intelligent individual, as well as an extraordinarily tactician; being able to convince Harrison Nash Wells to free him.
 * Master of deception/Manipulation: Mobius is highly skilled in the arts of deception and manipulation; as he was able to was able to deceive Harrison Nash Wells into believing that Mar Novu was the one who was trying to start the Crisis of 2019, prevent it from happening, Mobius also manipulate the latter into releasing him into Earth-1 so he gained a foothold into the entire multiverse.
 * Intimidation:  Mobius has a very intimidating presence to other heroes. Harbinger was proven to be visibly terrified when directly confronted by Mobius in person.

Weaknesses

 * Paragons: Mar Novu predicts the seven Paragons are the only individuals who can take Mobius down.

Equipment

 * Antimatter cannon: It was the ultimate weapon of mass destruction that produced the antimatter waves that the Anti-Monitor used in order to completely destroy the multiverse. This was later destroyed with the help of Team Flash, the Flash of Earth-90, Black Lightning, and Pariah.

Trivia

 * The Anti-Monitor's endgame was foreshadowed by Zoom's final scheme, seeking the destruction of the multiverse using a machine powered by speedsters running on it.
 * Though unlike Zoom, he didn't want to leave a single universe to survive, but rather rule the remaining antimatter universe alone.
 * Also unlike Zoom, he succeeded.
 * Because of Season 8 being focused around the Crisis on Infinite Earths crossover event, the Anti-Monitor is both the season antagonist of Arrow's final season and the main antagonist of the Crisis on Infinite Earths crossover event.

Behind the scenes

 * In the comics, the Anti-Monitor is a major antagonist and the main villain in Crisis on Infinite Earths. As The Monitor's evil antimatter counterpart, he sought to destroy the multiverse and replace it with his Antimatter Universe. Decades later in the New 52 storyline Forever Evil, he returned with a new design which the show's version is similar to, and later still in the Rebirth comics, his real name was revealed to be Mobius.
 * The Anti-Monitor is the most dangerous villain in the shared multiverse as he is with the largest body count. Except for the Paragons, he has murdered an infinite amount of people by destroying the multiverse with antimatter.
 * His physical appearance has been compared to other major pop culture villains, such as the Marvel villains Apocalypse and Thanos as well as the White Walkers and their leader in particular, the Night King, from Game of Thrones.