First Spinjitzu Master's Retribution:
A LEGO Ninjago Alternative Multiverse/Universe

Tamasui (later, the Overlord) and his son Sayangema lived in the First Realm together, but when the battle between the Oni and Dragons occurred, they and Hikari (FSM) fled to the Realm of Oceans and Seas (Later becoming Ninjago), where they used the elemental powers of Creation (Fire, Ice, Earth, Lightning) to create the island of Ninjago. 

As they took breaks from creating the landmass, Sayangema and Tamasui would venture out to see the surrounding area. As they did so, they came across a young Merlopian teenager named Avra who was the master of wind. She explained to Sayangema and Tamasui that her sister Naiad (later Nyad due to Ninjago language translation shifts) was more popular due to the fact that her elemental power was connected to the Water which was their home, compared to Avra who seemed like a foreigner with her Elemental powers. Nyad was always seen as the smarter one and the one who had a final say in things, while Avra was seen as less due to her attitude, which was built up by the unfair world she was living in. Tamasui and Sayangema would visit her frequently in secret from Hikari (who wanted to eradicate all other intelligent beings and didn’t realize there were beings underwater, as well as the fact that he believed all those associated with darkness were evil by default), growing closer in relationship with her, as though Avra was an adopted child. Though as Tamasui grew closer with Avra, he began to ignore his son. This—in turn—frustrated Sayangema, but unwilling to anger his father, he refused to speak up. 

However, when Hikari suggested making Wojira asleep, tensions arose. This is where the sisters started fighting. Whether this was true, everybody knew killing Wojira would cause the two sisters to lose their Elemental powers. Avra was ready to lose her powers, just to make sure that Wojira, who cannot really be blamed for her nature, has a merciful end, death. Nyad‌ secretly didn't want to lose her powers despite claiming she'd not mind if it was for justice, so she sided with Hikari forging the Storm and Wave amulets, which would divide a huge amount of Wojira’s powers into them, so she'd be too weak to awaken—anything but death. The two sisters made enemies, majority siding with Nyad and Hikari, and Sayangema and Tamasui siding with Avra. 

However, Hikari turned Avra into the Preeminent, cursing all future elemental masters of wind. This devastated Tamasui, as he had become a father figure towards Avra, and in his grief, he became increasingly more distant from Sayangema, pressuring him into extremely high standards. Sayangema was heavily uncomfortable with this.

When Hikari came after Tamasui and Sayangema, Tamasui hid his son within the Oni Temple as a means of protection, saying one final farewell, then battled Hikari for control over Ninjago. In a last-ditch effort to ‌defeat the First Spinjitzu Master, Tamasui merged with his element of darkness, losing his humanity partly, nearly forgetting about his son. 

When the land split, Tamasui realized his son was on the opposite landmass and could do nothing to bring him back. In his despair and devastation, he descended further into madness, vowing to take control of Ninjago to reunite with his child once again. In the meantime, Sayangema quickly realized that his father was no longer with him, and as a result became more and more resentful of him, to the point where he hates him almost completely.

As the years went by, Hikari created his two sons Wu and Garmadon using his element, refusing to explain Tamasui's true nature to them as they grew up, and posing him as pure evil in both legendary historical writings and myths, even going as far as erasing the memories of the father and son completely from his mind using his element of Destruction. Therefore, history continues as seen in canon with any information about Sayangema and Tamasui's relationship completely lost to time except for what remains in the memories of the two themselves.

Tamasui and Sayangema lived simply in Ninjago as the elemental masters of Darkness. However, when Hikari came, his philosophy of black and white thinking regarding all those who were associated with darkness and shadow being evil by default, was deeply resented by Tamasui. When Hikari threatened both Sayangema and Tamasui, Tamasui hid his son within a cave before taking on Hikari. In a last-ditch effort to ‌defeat the First Spinjitzu Master, Tamasui merged with his element of darkness, losing his humanity partly, nearly forgetting about his son. When the land split, Tamasui realized his son was trapped within the temple and could do nothing to bring him back. In his despair and devastation, he descended further into madness, vowing to take control of Ninjago to reunite with his child once again. In the meantime, Sayangema quickly realized that his father is no longer with him, and as a result, becomes more and more resentful of him, to the point where he hated him almost completely. 

 Tamasui came before the Oni could corrupt him and the FSM (Hikari). Tamasui raised Sayangema with Hikari together (non-romantically for clarification) then the Oni came. Tamasui fought the oni with the FSM but Tamasui (who dubbed himself the Overlord) and Sayangema got corrupted as a result. Tamasui raised his son Sayangema until he was 10 years old. Both before Tamasui’s corruption, and during it, he became emotionally distant from Sayangema, pressuring him to achieve extremely high standards, which Sayangema was heavily uncomfortable with. When Hikari came after Tamasui and Sayangema, Tamasui hid his son within a cave as a means of protection, then battled Hikari for control over Ninjago. As it turned out, Hikari was playing the Akurai family the whole time, believing all those who were associated with darkness and shadow were evil by default. In a last-ditch effort to ‌defeat the First Spinjitzu Master, Tamasui merged with his element of darkness, losing his humanity partly, nearly forgetting about his son. When the land split, Tamasui realized his son was trapped within that cave and could do nothing to bring him back. In his despair and devastation, he descended further into madness, vowing to take control of Ninjago to reunite with his child once again. In the meantime, Sayangema quickly realized that his father is no longer with him, and as a result, becomes more and more resentful of him, to the point where he hated him almost completely.

Tamasui is the soul of the void. Void was(n’t) an empty, timeless infinity and nothingness. It didn’t exist, as there wasn’t anything at all. Tamasui is driven insane from being everywhere forever all the time, as he can see the infinite possibilities that did and did not, will and will not, happen infinitely. Besides that, homesickness eternally flooded him, with grief, and by anger over something that never was, never is, and never will be. The Void. Yet Tamasui still feels it. He keeps seeing the nonexistent memories that make him desperate to reach it. He has the feeling that not even he himself can understand. Upon realizing he cannot destroy the infinite reality, he stops destroying (for a time), and unintentionally creates a son (Sayangema) as his first projection of his younger self, naming him Sayangema which means “Darling Echo [of Void]” in his ancient language. He hates him at first, acting cold towards him, but eventually softens up to him. 

Unfortunately, before they could grow closer, the First Spinjitzu Master invaded the pocket realm, spiraling into a battle that causes Tamasui and Sayangema to be lost among the realms. As we see in canon, FSM splits the island tethering him to the Dark Island, and separating him from Sayangema. This separation drives him into grief and madness over losing his child, as well as the void that never was, never is, and never will be. Infinite times, he tries to create a vessel that he would use to achieve distress amongst the living and/or achieve potential Peace in the Dark. He’s caused distress an infinite amount of times, but in the end, came nowhere close to what he ultimately wanted, yet he keeps pushing to achieve something that never was, never is, and never will be. Sayangema, meanwhile, believes his father had died, and vows to reunite with him, secretly resenting him as he believed that it was Tamasui's fault they were separated, and in his grief, descends into madness.