Yhwach's personality was such that when he wanted someone to become part of his empire he would put in great effort to do so even if he had to be forceful. Aizen due to Keiko's manipulative suggestion was eager to learn about Yhwach and his powers to become more supreme and perfect his existence.
Aizen secretly and slowly observed Yhwach from far away and started coming in his good grace and erasing any hint of betrayal from Yhwach's mind.
Along with his manipulative plan, Aizen also started secretly performing experiments on Yhwach's armies which were originally Quincy but has evolved into a higher form due to the powers which were bestowed by Yhwah using the upgraded Hogyoku which was created with the combination of both Hogyoku he has created along with the one Keiko let his take from Rukia.
Yhwach recovered all his memories and powers which were taken after 1000 years discovered that the Shinigami clan which he has let behind were getting more powerful after getting wind of his possible revival, Unlike other people he had the memories of the soul kind which meant that he knew more things which exist outside the boundaries of this world, thus after discovering Hogyoku took great interest in it, almost to the point to take in the person who had embedded Hogyoku into himself as his subordinate, Yhwach with his experience in the lesser god realm was a very paranoid person he as a means of making sure that subordinates around him don't revolt against him placed a little soul piece of himself into everyone around him to observe them in shadows. Taking the same measure with the holder of the Hogyoku Yhwach placed a piece of his soul into Aizen too under the pretense of the granting more powers which everyone wanted.
Aizen under the watchful hidden eyes of Yhwach continue doing experiments on his subordinated meanwhile, Yhwach formulated a plan to use Aizen to his advantage in the upcoming fight against Soul Society especially Yamamoto who's Zanpakuto was very special as it consisted a spark of the origin flam which would burn all creation with the realm it was used in, But Yamamoto alone being a mortal was unable to utilize it completely but even so, Yhwach didn't want to directly face such variable which if used in an efficient ways could potentially harm him or worst erase his. Thus with this in mind, he started formulating this plan.
The day of the battle started, Yhwach with his plan to commence the battle against soul society. While ordering Aizen to leave from the battle in the human world to act as a distraction which he knew wasn't going to happen took aside one of his subordinates who using his powers changed into Yhwach completely while using the in-depth research which Aizen has done for him on his subordinates Yhwach developed a method of his own via which he linked all the soul pieces which he has granted to his subordinates with each other which also included Aizen.
With all preparations done, Yhwach decided to take one last step by taking in Uryu as his heir, which he did to have a vessel that would be left behind for himself in this mortal world in case he after absorbing is bound by world laws in anyways.
Thus the battle commences, Yhwach from the shadows observed Aizen releasing his Shikai in the human world and trapping everyone there from the people he has sent with Aizen to give a sense of Aizen's plan working perfectly. Ignoring the plea of the people who he has sent with Aizen, he observed that Battle between Yamamoto and his subordinate Royal Lloyd.
Aizen in the meanwhile in the shadow watched the battle, waiting for Yhwach to get weaker slowly, seeing this using the link created between the soul pieces. Yhwach lowered Royal Lloyd's powers by slowly transferring his powers to other subordinates which were fighting against Captains of other squads. Minimizing any kind of suspicious slowly, Yhwach wanted to bring out all the rats which were hiding to surprise attack him.
Yhwach seeing the situation slowly develope moved to observe the Human world but all he saw was a destroyed and discriminated force of both sides completely. Thinking that there is some kind of method used by Shinigami to achieve mutual destruction, Yhwach not caring about the situation observed the battle in soul society. Yhwach observed that now along with Yamamoto, Ichibe was also fighting against Royal Lloyd seeing the situation go out of hand as he originally thought that Ichibe and the royal guard will not interfere with the battle until he reaches the Royal palace, Using the link between themself, the Wandenreich manage to hold on and create a stalemate situation between the two forces.
Yhwach observed Aizen as the lateral with his full confidence jumped into the battle between the 3, Yamamoto, Ichibe, and the fake Yhwach. Aizen pretending to be on the Yhwach side started fighting Ichibe who according to Aizen was much more suitable to fight against as per his powers.
With this fight commence with slowly the side of fake Yhwach and Aizen coming on top, Yamamoto who so far was fighting the way which Keiko has taught him without wasting his powers and using Kido arts while minimizing the Reiatsu output thus storing the power to wait for Keiko to finish in the human world and arrive here as backup. With Aizen's arrival, he along with everyone was certain that Keiko too will come here soon and thus with his arrival the battle will end with their side winning.
Seeing the change in the balance and feeling the suppression, Yamamoto with the signal of Ichibe released his Shikai completely.
Seeing this as a change Aizen who was fighting beside fake Yhwach turned to Yhach and used his complete powers and attacked Yhwach from behind.
Yhwach seeing this transferred all the energy which Royal Lloyd had to other people which were linked with Royal Lloyd due to soul network which he had created thus giving them a boost in power as they were fighting with Captains.
With Royal Lloyd's death, before dying the truth about him being fake Yhwach came to light. Yhwach without wasting time channeled Aizen's energy into his subordinates thus crippling his power.
Yamamoto who had already released his Shikai before Aizen's betrayal quickly released his Bankai after seeing the real Yhwach enter the fight. Meanwhile, Ichibe also released his bankai sealing the entire area to trap Yhwach when he is distracted.
Yhwach with his control over the soul piece, channeled all the energy out of Aizen thus incapacitating him while releasing his power of Medallion to seal the bankai of both Yamamoto and Ichibe but due to Yamamoto not wasting powers earlier and using Kido arts to fight through, fails to seal it while Ichibe being relatively weaker than Yamamoto failed and had his Bankai sealed.
With Only real threat of Yamamoto hanging over his head, Yhwach signaled his subordinated to commence the backup plan in case, he fails to steal Yamamoto's Bankai. Yhwach's subordinate bring fought the hybrid of a newer version of an Arrancar made after Wonderweiss Margela which Aizen had created to trap Yamamoto's powers in. Thus avoiding direct confentration.
With wave of his hand the modified hybrid came out through the sealed space. By before it could do anything a heavy death aura fell on the entire soul society. Yhwach using his godly connection tried to find the source but heard the voice like of an ancient monster right next to his ears.
"Are you ready to dance?"