Arc City + Sea of Quantum:
After losing Kiana, Mei became so depressed that she willingly betrayed Hyperion and cooperated with World Serpent. Obito promised he would protect Kiana at the cost of Mei serving as a spy for him and Madara. She was also injected with the Rinne Serum and trained by Obito to reach 2-tomoe Sharingan. After being informed by Obito that Kiana was hiding in Arc City, Hyperion immediately headed to the said city and worked to find Kiana.
Unfortunately, while fulfilling his promise, Obito suddenly received an important mission from Madara. Having no choice, he had to leave Kiana alone and carried out his mission. Obito went to the Sea of Quanta to investigate suspicious deeds of the Gray Serpent there. He later found out Gray Serpent was planning to bring Kevin Kaslana back, replacing Madara with Kevin as the true Sire of World Serpent. During this trip, the masked Uchiha encountered a girl named Seele Vollerei, who was Kevin Kaslana's servant. In the end, after stealing Bronya's Herrscher Core, Kevin successfully escaped the Sea of Quanta
Failing in stopping Kevin's return, Obito intended to use Kamui to bring Seele along with him back to reality. However, Seele refused to go with him and she begged him to save Bronya. Seeing Seele's warm heart and kindness like himself in the past, Obito decided to give her the Gem of Serenity which he had taken from Kiana in their previous battle. Seele ascended to Herrscher of Death and used her new powers to save Bronya.
Returning back to Arc City, Mei faced Raven while finding Kiana and thus had a battle with the World Serpent's agent. Nevertheless, they soon found out their enemy also possessed Sharingan and realized they both were on the same side. Raven then promised to help Mei find and protect Kiana.
Raven vs Kiana: Unlike in the canon story, Raven without using any dirty tricks, overwhelmed Kiana with the power of Mangekyou Sharingan. Raven at first used Tsukuyomi to capture Kiana but failed as Kiana with the help of Fu Hua broke out of the Genjutsu. In the battle, Raven won by using Susanoo to suppress the Herrscher.
After that, Everything continued like in the main story. Kiana used her Herrscher power to save Arc city from the Honkai Reactor of World Serpent. She later went unconscious and was brought to Schicksal's base by Durandal. Mei tried to stop Schicksal's strongest Valkyrie but was beaten due to their gap of power. Mei then fell into depression because of her uselessness. In anger, she awoke 3-tome Sharingan.
Battle for the throne: Kevin vs Madara
Kevin finally returned to World Serpent and declared that the throne belonged to him. World Serpent was divided into two factions, those who supported Kevin and the others who wanted Madara to stay as the Sire. In the end, the conflict was solved by Gray Serpent as he suggested a battle between two men. If Madara could defeat Kevin, he could continue being their Sire. On the other hand, if Madara failed, he had to leave the organization and return the throne to Kevin. The fierce battle occurred and lasted for 3 days. In that battle, Madara went all out against Kevin and used the strongest attacks he had: Limbo, Perfect Susanoo, Planetary Desvation, and Shattered Heaven,... But in the end, he still lost and died. However, Kevin was also heavily injured and it would take him months to recover from the battle.
Rescue Operation of Kiana:
Since Kevin was heavily injured after fighting Madara, he let Gray Serpent hold the power temporarily in World Serpent. Gray Serpent agreed to help Obito and Raven rescue 2nd Herrscher and thus held an assault against Schicksal. But in the middle of the battle, both Gray Serpent and Otto got to know that Herrscher of Ice had awoken in the Coral city of the Philippines. Having a common enemy, the two organizations decided to cease the war and make an alliance to deal with a new threat. Taking advantage of the chaos, Kiana had escaped before with no one knowing.
Nagozara Arc:
Post the war, Mei and Raven wenttoNagozara as they recorded traces of Herrscher's energy within the city. While searching for Kiana, Mei asked Raven about how she had awoken Mangekyou Sharingan and was terrified when she got the answer. In order to awaken Mangekyou Sharingan, Raven killed the orphans who she used to love.
They eventually found Kiana but Mei later learned that her beloved just had 3 months left to live. In depression, Mei was told by Kevin that if she became Herrscher of Thunder once again, Kiana's life would be saved. Fighting Husk-Nihulusk, Mei used her strongest technique, Kirin to destroy the Imaginary guardian but it was recreated right after that. Exhausted and out of chakra, Mei was almost killed if it wasn't for the sacrifice of Benares. The Honkai dragon used its body as a shield to protect her and was killed in the process. A heart filled with hatred, Mei awoke her Mangekyou Sharingan and also regained Herrscher of Thunder's authority. She slayed Husk-Nihulusk with one slash only and burned it to ashes with Amaterasu.
Mei Raiden then chooses to join World Serpent with Raven but Kiana tried to stop her. Having no choice, Mei was forced to take down Kiana in order to protect her.
Herrscher of Ice's arc:
Mei, Raven, Owl, Durandal, and Rita went to Coral City to capture Herrscher of Ice. the 5th Herrscher turned out to be Ana Schariac, Owl's love, and thus Owl betrayed his allies to save her. Before they could leave Coral City, Mei with her incredible speed blitzed the two and stabbed her sword at Tianwu Chen directly in his heart. Owl fell down and witnessed his love being killed by Durandal with Black Abyss White Flower. Tianwu Chen with his extreme anger, who was thought to get nothing from the Rinne Serum injected into him, suddenly awoken his Rinnegan. He was also granted Herrscher of Star's power by the Will of Honkai. Revenging for Ana, Owl used Deva Path power enhanced with Herrscher of Star's authority, creating an Almighty Push on a large scale. The strike was strong enough to wipe out the whole Philippines and killed all of its population, casualty skyrocketed with the deaths of more than 100 million people.
Owl's power terrified everyone. He easily killed Rita and overwhelmed the 3 others. At that moment, Owl was the strongest being in the world. Herrscher of Star's authority combined with Rinnegan made him nearly invincible. He easily destroyed Mei and Raven's Susanoo with a single strike. In the end, Durandal lost consciousness and let the beast inside her rampage. It was revealed that Durandal was the Jinchuriki of Nine-Tails after Kurama's death when Naruto used Baryon Mode. Even so, she still stood no chance against Owl as he once again defeated her. However, Anna's spirit had a talk with Owl and calmed him down. She convinced him to reunite with her in the afterlife. Thus, Owl stopped defending himself and let Mei kill him with her final blow.
At the end of the battle, Mei found out that Raven was dying and her comrade gave her the final gift. Before she had last breath, Raven took her eyes out and gave them to Mei, asking her to use them to awaken Eternal Mangekyou Sharingan. Later, Mei was given a mission by Gray Serpent to keep Owl's Rinnegan from falling into the hand of Otto. Yet, it was too late to do anything. Durandal had escaped after stealing the pair of Rinnegan from Owl's body. Otto wanted to use them for research purposes.
Herrscher of Sentience's awakening:
When Durandal returned to Schicksal's HQ, she was suddenly attacked by an unknown enemy. The said enemy was Herrscher of Sentience taking over Fu Hua's body. Back to the battle against Herrscher of the Void, Fu Hua was killed by Otto but he accidentally shot at her left eye and destroyed it. Losing one of her eyes, the Herrcher revenged by knocking down Durandal and taking a Rinnegan to fill her left eye.
She cooperated with Obito, the only person who possessed Rinnegan at that moment. She asked him to teach her to use the new left eye. The Herrscher soon advanced and learned all 6 Six Paths abilities granted by Rinnegan. In addition, she also was able to sue Limbo - Madara's Rinnegan technique. It was later revealed by Obito that the Rinne Serum injected in Owl contained Madara's blood. He created multiple types of Rinne Serum with the blood of different Uchihas. Mei's was Sasuke's blood while Raven's was Itachi's. It also explained how Mei could awaken the Eternal Mangekyou Sharingan after transplanting Raven's eyes.
After a few days, Kiana, Bronya, and Seele arrived at Mount Tainxuan to discover Fu Hua's past. There, they had a one-sided battle against Herrscher of Sentience. Although it was 3 Herrschers against one, they were played by the Sentience as she constantly spammed Rinnegan's abilities. Bronya and Seele soon were defeated by her and sealed with 2 Planetary Devastations. The only one left was Kiana while struggling against Herrscher's tricks. Herrscher of Sentience decided to crush the Kaslana as she cast Shattered Heaven and summoned a mountainous-sized meteorite from space. With Fu Hua's help, Kiana successfully performed the edge of Taixuan and defeated her enemy. Fu Hua regained her body but there was a consequence for it. Since Herrscher of Sentience was sealed and so was her authority, Fu Hua couldn't use Herrscher's energy to transfer to Chakra. For that reason, her Rinnegan always drained her Chakra reserves because it was turned on 24/7. Fu Hua was unable to inactivate her damn eye and it soon became a pain in the ass.