Stein's Nest

"I was once one of Ajuka Beelzebub lead scientists. I proposed to him the idea of creating a pseudo-Demon King. No, a demon king that would surpass the former Demon Kings but he threw my idea out, and I was forced to go back working in the Rating Game sector. Well damn Ajuka Beelzebub and damn his entire science center. I created something much better than a simple board game for rich devil children to play, I created a God."

- Stein's Resolve The Stein Nest was a group of antagonists that acted as the major antagonists in Life 3 and 4. Acting as the concluding antagonists for the My New Life as a Devil of Leviathan Arc. The Nest was created by the Stray Devil Doctor Stein and was later official disbanded after his death, during Sona's team mission to exterminate the Nest.

Summary
The Stein Nest was noted to be too powerful for such a young Nest. The Nest only being a year old, it was founded by Doctor Stein, a servant to the Beelzebub family and a head scientist of Ajuka Beelzebub. He created the Nest with the intent of using the other stray devils as food to power up his creation, a being known as Havoc who he called the Next Generation Demon King.

The story goes as follow: Stein was originally a head researched for Ajuka, who claimed that Stein was very good at his job as a Rating Game researcher, however Stein later decided to switch to his research in bio-mechanics with his plan to construct a new Demon King. Ajuka later rejects the project and Stein is sent back to work on Rating Game Research however he runs away, taking his research with him. He later heads to Germany where he acquires some Stray Devil however in order to stay on the run he continues to jump from country to country later landing in Japan where he plans to destroy the sisters of Ajuka's best friends as revenge for him rejecting his work.

In the report, it is later learned that Stein was hypnotizing and experimenting on humans in order to use them as slaves, stealing 56 in total, but luckily his experimentation wasn't permanent as Ajuka and Gray were able to completely undo all the damage he did and the human's memories were erased and they were all returned back to their homes.

Stein's Nest Members
Havoc (Deceased): The primary goal of Stein was to create a Demon King and use the other members as food to feed both Havoc and his power. He planned to use Havoc's strength to kill Rias and Sona and their households so he could take over the Japan area. However the creature was killed by the combined strength of Sona and her peerage.

Stein (Deceased): the Leader of the Nest who lacked any and all battle capabilities however he was able to create a creature known as Havoc, even sacrificing his own life span to power-up himself. He was planning on using the other members' of the Nest DNA as food for his creation. He was killed

Amera (Defected): The most gluttonous of the Nest, a stray devil who is noted to be the most powerful. She killed Saji and Nimura at the initial start of the series which causes the two to be reincarnated as devils. She wasn't present during the destruction of the Stein's Nest but was watching the battle from afar. After learning of Ichiro's powers she decides she wants to eat him. She possesses the sacred gear [trick vanish].

Yui (Deceased): The third strongest of the Nest with equal strength to Marcus during the invasion of Stein's Nest. Not much is known about her other than she possessed powerful lightning abilities that she used on Meguri, Tsubasa, and Saji. She was killed by a sneak attack by a insane Ichiro, who cut off her head while she was standing on top of a large tree trunk before kicking her body to the ground. It it assumed that her body was destroyed by Ichiro's Dragon of Annihilation attack due to Gray Dantalion only finding her horns which had scorch marks on them.

Marcus (Defected): The second strongest of a Nest, a evil and manipulative individual who killed his other teammates after he decided to ally himself with Raynare and her group. He killed his master due to jealously, after learning he was marrying a woman Marcus wanted, however the woman didn't want him thus in blind outrage he kills her two. Ichiro later jokingly calls Marcus a hypocrite, while he "rattles on" about how Ichiro is a slave and not free, he's a slave to his own love and hunger (a reference to him becoming a Grim). Marcus became a grim after he ate their bodies before traveling to the human world and eating humans. He was the one who completely annihilated the Nest, having killed all the other stray devils aside from Yui and Amera. However sensing both Diehauser's and Gray's introduction to the area instead of fighting against the Sitri peerage he decided to leave.

Unnamed Stray Devil Girl (Deceased): A unnamed stray devil girl who masqueraded as a 1st year Kuoh Academy student but later revealed to be a stray devil when she tried to attack Saji's group. Sona even noticed that she stopped attending classes after their mission. She was powerful enough to fight both Tsubasa and Saji head-on with immense strength. She was later killed by Havoc, the creature created by Stein.

Unnamed Stray Devil Boy (Deceased): A unnamed stray devil boy who masqueraded as a 1st year Kuoh Academy student but was later revealed to be a stray when he allied with the girl and attacked Saji's group. He wielded a axe and had immense speed capable of rivaling Meguri. He was one of Marcus's victims.

6 Unnamed Strays (deceased): Sona, Tsubaki, and Nimura fought against six stray devils and Sona promised to allow one to live if he explained Havoc and what was happening. All six of them were the victims of Marcus, each being stabbed with a spear of light.

Fights
Meguri v.s. Unnamed Stray Devil Boy (Meguri's win, she restrains him with her magic)

Tsubasa and Saji v.s. Unnamed Devil Girl (Tsubasa's and Saji's win, she retreats)

Meguri, Tsubasa, and Saji v.s. Hypnotized Humans (Meguri's group winswins)

Ichiro, Reya, and Momo v.s. Hypnotized Humans (Ichiro's group wins)

Ichiro v.s. Marcus (Ichiro's Win)

Sona, Tsubaki, and Nimura v.s. 6 Unnamed Strays (Sona's group Wins)

Ichiro v.s. Yui (Ichiro's Win, decapitates her)

Sona Sitri Peerage and Ichiro v.s. Havoc (Sona's Group Wins)

Groups for Operation
These groups were the initial groups that were meant to fight the Stein's Nest, being formed by Ichiro to level-in at all the strength and weaknesses of Sona's newest servants and to each fulfill a role in the mission.

Sona's Group
Sona, Nimura, and Tsubaki: “Because while I dislike you and Rias’s overreliance on magic, I understand that for offensive magic it can be very useful. And while Tsubaki’s offensive magic might not be as powerful as yours, in terms of defense however her magic will be good at backing up your offense if you two engage in a fight. And Nimura...she's a decent close range fighter and at least she didn't cry as badly as Saji when I hit her."

Saji's Group
Saji, Tsubasa, and Meguri: "It's obvious that Yura can be the closed range fighter of the squad. If we add that with Saji's sacred gear ability to drain energy from the enemy and add it to Meguri’s fast sword technique who is also quite skilled in reverse magic then the team will be perfect for backup and support. But, I don't expect Yura to use her close-range combat and strength as a mean for fighting but as a mode of defense. It would be better to use this as a way to protect Meguri and Saji as they are transferring energy. Their team isn't the main focus though and is only necessary if my team fails to complete their assignment."

Ichiro's Group
Ichiro, Momo, and Reya: "My team will work as the scouts. Our goal is to find the enemy using my soul hounds which won't be disappearing any time soon due to the night. With both bishops acting as my backup and supporting my search. If we encounter enemies that we can’t defeat then we will hold them off long enough for Saji’s team to arrive. However, our status as the scouts of the mission is much more important because we will be focusing on finding the stray devil bases. They can help distract the opponent long enough for me to capture the stray devils. When I have them captured Meguri or Momo will be able to signal you and the others and you can come finish the stray devils off."

Legacy
As Marcus finds Stein in the forest before he kills him he grants Stein one last request and Stein states that his final request isn't his life but for his research, that he saved on a hard drive, to be released to the black market on the dark web, Marcus smiles and complies with his request thus it can be assumed that he uploaded all of the information on the Dark Web this is later confirmed during his second fight with Ichiro where he reveals that the Next Generation Demon King will make a return.Thus in some way, Stein's plan was a success.

It is also revealed by Gray that the old man who lived in the forest was left relatively unharmed, while he was one of the humans affected by Doctor Stein's experiments he has made a clean recovery.

In the report it also details about Ichiro forming the initial teams, how Ichiro captured the strays alive and saved the life of Sona's servants, and even how Ichiro was the one who unleashed the attack that destroyed Havoc. Because of Sona's report, Ichiro is scouted by the Demon Council to become their Next Generation Knight of Harbinger.
 * Sona intentionally leaves Ichiro's psychotic break out of the report stating that because he didn't hurt anyone on her team and actually helped save Saji and the others it wasn't important.

Trivia

 * This is the first organization I have created with so many unnamed and unexplored character.
 * This was originally written to be a basic two-part series that was never explored later in the series however I added elements such as Marcus and the Stray Devil Research being released to actually give the 2-part fight a actual impact in the story.
 * Despite her fight being off-screen technically Sona's group defeated the most stray devils.
 * Ichiro and Marcus are the only two who actually killed stray devils during the operation. While Marcus killed them and ate pieces of their flesh for his own gain, Ichiro killed Yua due to fighting a psychotic break.