Swiftly, the secret soldiers of Garasu no machi entered the spirit world and divided themselves in a way that created a big decagon, then they used the newest glassbending technology: one way glass. This glass has the ability to let the person from one side see everything, and the person from the opposite side see nothing, they all bended small curves above the ground, and made the top out of this special glass, so they can look around without being noticed, with the terrain still looking natural. It didn't stop there, they created a smaller decagon inside the big one, so they can get a good view of everything from every angle, and they connected all of the 200 chambers, in case one needs backup, or they need to escape to the hidden tunnel they dug. Yin and the rest stayed in the secret room, the one that had the 200 screens, looking at them waiting for anything to happen, they all decided they'd stay there for 3 days, if norhing happens, they need to change the plan.
Just as day 1 was ending, and everyone slept except for Yin, who's eyes have started to close and he began to fade away, he noticed movement on one of the screens, so to wake everyone up, he spun cold air around their faces until they all woke and he pointed toward the group of people he's seeing, and after a while, it was revealed that they were chopping plants, Avatar quickly said "code 4" 4 stands for the 4th letter of the alphabet, D, which is what the word "division" starts with, code 4 is where secret service splits, for each target only 1 soldier follows, and the rest observe from far away, Yin wanted to find out what's going on so he ordered them to execute the code so they could see where them and the plants they chopped are heading to. "Remember, you're not machines, you're humans with one life and with feelings, if you ever fear for your lives you can back away, I'm coming in a few minutes to join you"
Avatar Yin didn't want his soldiers to lose their emotions, these men and women are really brave, and he wanted to always remind them that it's okay to break down from time to time, after all, they're humans. He got up from his seat and told Akira, Hiroku, and Naomi to keep a close eye on the cameras before he quickly and quietly bended his way toward Republic city, it didn't take too much time to arrive, he knew what bending to use at what time to save the most time. When he reached the place he last saw the soldiers in, he made the earth necklace come out, it makes it easier to feel the eaethnending energy and detect earthbenders around, and that's what he did, he found them, then went to join them. He taught his soldiers all they know about stealth, and in fact, he had a couple of tricks up his sleeve that he kept as a secret, to surprise everyone when it calls for it, he learned that it's better to always have something unheard of, it's good to do the unexpected. Avatar Yin's special move, other than the scorpion sting of course, is....well, why should I spoil it? Waiting for it would be better, but all I can say is, even I, the writer, didn't expect it.
The Republic city scientists reached a dead dark end in a street that's unheard of, then one of them rubbed his leg around and earthbended the ground to make it open, in they went. Avatar Yin told the soldiers to stay, as he goes on his own, it was risky, but he promised not to let anything slide anymore, and he's keeping that promise. He slowly opened the secret area a while after, to make sure the other guys aren't near it. He went down, there was a stairway leading deep down, with every step he took he airbended a bit so his steps were almost completely silent. He found himself needing to go through a tunnel that had plenty of guards, time for another of his signature moves; he bended the one way glass around himself, and started walking through them, sound waves travel through air, so with each step he took, he bended air in a way where the sound generated by his walk will never reach their ears, he was fully invisible and unheard, one way glass us glass from the inside and mirror from the outside, but since the soldiers were symmetrically aligned, ant he place was almost pitch black, there was little to worry about, all the way through he couldn't help but grin, he spent so much time training on how to completely control the air around him and stop sound waves from reaching anyone, and his work is paying off. After what seemed like an hour, he reached the headquarter of the facility, where all operations were being performed, and he saw how the plants get put into a glass tube that has 2 things at the top and bottom of it that sucked all the electricity from the plants, he then saw the mayor coming and talking with the scientists. Disappointment filled his eyes when he saw the mayor, Yin thought that he was a trustworthy person, apparently not. He decided to leave and think about how to go about it, should he capture them all, or confront the mayor, there were tons of options, but he decided to leave a small gift before leaving, after the second badge was entered the tube and the electricity just started to get sucked out of it, he opened the glass tube by glassbending it, and that caused a combustion, he then left swiftly. Against the odds, his spying mission was successful.
Next morning, he held a meeting with Naomi, Hiroku and Akira
"-We have footage as evidence, but I don't know how to peacefully go about it: invading Republic city does bot sound peaceful at all, confronting the mayor might get him to attack Garasu no machi, none of the options are good, it's like to fix something we need to break some stuff on the way (Yin)
- the best and most logical thing to do is to confront the mayor, if he refuses to stop, then I can promise that Garasu no machi will have alliance with the air nomads (Naomi)
-and the fire nation (Akira)
-and the water tribes (Hiroku)"
And with backup reassured, Yin went and stormed into the Mayor's office next day.
"Avatar!" The mayor said surprise
Yin sat down next to him and airbended the footage toward his face, so he would see it clearly, after looking at it, he talked "so, care to explain?" Yin said not angry at all, just disappointment. The mayor grinned and replied:
-I knew it was only a matter of time until you find out. There's no need to hide anything since you now saw it, me and my team have been extracting energy from plants from the spirit world to create really powerful weapons, you're not going to try and stop us, will you?" And directly after he finished talk8ng he pressed a button under his table that launched troupes from every direction, ready to fire at his signal. The avatar stood up slowly and smiled saying "you really think you or your troupes can beat Yin?" The fire ring and earth necklace came out and started glowing red and green, and so did the avatar's eyes, he then tilted his head and said "we'll meet again, very soon. And there would be a lot of glowing while I'm here, mark my words" then fire surrounded him and started expanding, by the time it vanished, so did the avatar. Yin training in secret for 10 years made everyone heavily underestimated him, what he showed until now is still nothing of what he can do. After leaving Yin started preparing a plan for all nations to attack republic city.
"This is how we're going to do it; this is the map of the world

Garasu no machi and the earth kingdom's soldiers will attack from their side, air nomads will split, northern nomads will be going with the north pole, the western ones will join the fire nation, the southern ones will go with the south pole and the eastern ones will go with us, some of the earth kingdom's soldiers will spread throughout the perimeter of Republic city, and create a big wall, so that they're surrounded from every direction and have no way to escape without being seen. We don't want tp hurt the citizens, we just want to get rid of the mayor and every army member under him, so that Republic city can have a fresh start. Every TV station will be cut and replaced with an emergency warning for all Republic city's people asking them to head to the underground train station, where they would meet Zaofu's metalbenders and get taken away to a safe haven until we're done. If a citizen refuses to leave, that's on them, not us. We will warn the mayor of attacking if he doesn't surround, and if he refuses we'll immediately start attacking, remember, without killing anyone, just making them faint, and this is how you're all doing it; for firebenders, you're going to fire multiple continuous small boulders of fire towards your opponent and in a few second they'll be on the ground. For waterbenders, you're going to create a water sphere around one's head until they lose consciousness, airbenders will also be creating a sphere, but they'll be taking out the air from people's lungs and put it in that sphere until they faint, and for earthbenders, we're going to be creating a rock sphere around their heads then shrink it to remove all breathing room, in a few moments they will not be able to resist anymore. I'm telling you about this so when you return you can teach your people about it, it's not hard at all. Now I'm not a fool, and I'm 100% sure republic city has been creating different sorts of weapons, and we need to beware of them, if a weapon you've never seen before starts being used, as soon as they use it, you need to report it to the rest of us so that we have a faint idea of what we could possibly face" after that, Naomi, akira and Hiroku parted and everyone went to tell their people about the plan, they only had 2 days to prepare. Late at night, while Yin was meditating, Raava got out to speak with him
"Don't forget, you might be able to enter the avatar state whenever you want to, but you're still not able to get out of it whenever you want to yet, you still don't have control over yourself, especially if you stay in the state for a long period of time"
While Yin was busy talking to Raava, Republic city's force went from cutting some plants a night to cutting a whole field per hour, they knew what's coming, and they were preparing themselves too.