Bosacius was born in the middle of a storm, where lightning fell from the sky with booming thunder accompanying it. He was born at a time where gods were plentiful in the land of Liyue, before the time of Archons, before Teyvat was engulfed in war.
In the time of gods, there was a large demand for adepti who could protect those gods and their worshippers from their enemies. These adepti would later become 'yakshas'.
Bosacius was one such adeptus. He sold his services to the highest bidder, always moving from one master to the next. During the Archon War, he was popular amongst the gods vying for the Archon's throne. But when he saw Morax for the first time, he was in awe at the god's power. He left his master and offered his services to Morax, the latter only convinced to accept Bosacius at Guizhong's bidding.
He fought for Morax for the rest of the Archon War. But after Morax won, Bosacius found himself lost. Would Morax have any need of his services in a time of peace?
When the call for yakshas came, Bosacius was one of the first to sign up.
He met Pervases and became friends with him. He fought with Pervases on the battlefield, before he became part of the 'Five Yakshas'. Pervases was always by his side and he was someone Bosacius could call his brother in arms. They would share meals of Grilled Ticker Fish while boasting of their exploits over the campfire.
When Pervases perished on the battlefield, Bosacius offered a meal of Grilled Ticker Fish to his grave. It was the last time Bosacius ate Grilled Ticker Fish.
Bosacius was quick to move on from Pervases death. The growing number of demons and the shrinking number of yakshas meant that there was little time for rest. And in a blink of an eye, Bosacius found himself leading the yakshas, becoming the strongest yaksha even amongst the 'Five Yaksha'. He did not gloat in his glory, instead he tried his best to lead and when he looked back and saw everyone willingly following him, he marched forward.
Even when he became the leader of the yakshas, Bosacius still joined in bonding with his fellow yakshas.
He would regale tales of life before the Archon War to Indarias. He would help Bonanus play pranks on his fellow adepti, always laughing with her when their pranks succeeded. He would talk philosophy with Menogias, oftentimes having intense debates over the topic of the day. He would watch over Alatus as he napped, sometimes even joining him when he was tempted by the relaxing breeze and sleepy clouds in the sky.
When Indarias and Bonanus married, it was one of the happiest events that Bosacius had ever experienced. He shed tears in his happiness, leading to all of the 'Five Yakshas', even Alatus, and Morax, teasing him about it.
In a time where the land was engulfed by strife and suffering beneath the banner of peace, Bosacius embraced the moments of lull where he did not have to march to the battlefield and wonder which of his brothers and sisters would die that day, where he did not have to put a yaksha out of their misery when they succumbed to the growing corruption within them, where he did not have to stay strong as tears fell when one of their own perished.
(But good things were not meant to last forever.)
When Indarias did not return from her regular patrol, Bosacius grew worried for his sister. When the fires of the adeptal realm went out, Bosacius ordered search parties to be sent out. When Menogias came to him, telling him that he had found Indarias with a face full of grief, Bosacius felt his hope die. When Bonanus screamed and cried in her grief as she cradled Indarias' body close to her, Bosacius tried to steel his heart to separate the two but he failed.
It was only by Morax's arrival that Bosacius was mercifully spared of the cruel task to separate Bonanus and Indarias.
After Indarias' funeral, Bonanus was never the same again. Bosacius tried to lessen her duties but Bonanus always found a way to go against him. He tried to stop her, only for Bonanus to ignore him and leave the adeptal realm. He tried to search for her from dawn to dusk, even when the sun rose the next day.
It was only when Alatus called his name, when he moved towards where Alatus was, that he found both Bonanus and Menogias dead. Bosacius fell to his knees at the sight of his dead brother and sister.
A year later, every yaksha except Bosacius and Alatus were dead.
Where did it all go wrong? What happened? Why did it become like this? Was it his fault? Bosacius begged and pleaded for the answers to his questions but no one answered.
Time continued to tick by and the yakshas continued their duties, even when only two remained in the whole of Liyue to fight off the neverending tide of demons. Time continued to tick by and Bosacius continued to fight, never once giving in and never once taking a rest. Time continued to tick by, until one day, Bosacius woke up and realised that he could no longer remember the yakshas he had once fought together with.
He realised that he had forgotten the faces of the brothers and sisters who died before him.
Once he realised that his memories were being consumed by the madness, he decided that he would not burden Alatus. He left the adeptal realm and set off in a random direction. It did not matter where he would go, as long as he was far from his only brother, he would not cause trouble to him. And no matter where he went, there would always be demons to fight.
Bosacius left his treasures sealed in a ruin, too cowardly to face Alatus to hand him his will, but not fearful enough to write it down on a stone tablet for the next person to find. With one final glance at the last belongings he once held, he left to wander once more.
The madness devoured the faces of his friends and siblings. It devoured everything and left an empty shell of an adeptus. Bosacius would often find himself asking, to no one, 'Who am I?'.
It was fate that brought Bosacius to the Chasm when Khaenri'ah unleashed its monstrosities upon the world. It was fate that allowed Bosacius to lead the Millelith to fight back against the onslaught. It was fate that brought an artefact that had the power to twist the world around them and create a separate realm to seal the deadly monsters. It was fate that Bosacius was there to power the artefact with a human, trapping both them and the monsters in the realm.
How ironic that Bosacius was never the kind of person to believe in something like fate.
Even with his mind gone, Bosacius was glad that his siblings were here by his side, even if they would be trapped here for all eternity.
But that was not right, was it?
He was glad that he was not alone in his final moments, even as he had trouble breathing, even when he could no longer stand without support, even when he knew he was selfish for being glad, he rejoiced at the warmth of Alatus by his side.
But that was not right, was it?
It was when he could barely take a breath without the pain of drowning, when he could barely open his eyes to stare at his surroundings without the world blurring, that he realised his siblings were not there. And yet, he was glad he was not alone even if Boyang was not one of his family.
Even as he took his last breath, Bosacius was still a yaksha. He gave his life to protect the people of Liyue, he still had his contract with Morax. And with his last breath, he and Boyang used the artefact one last time to bury the chaotic realm with them in it.
Like a lightning that lasted but only a fleeting moment, the Yaksha of Lightning remained forgotten by the people he had sworn to protect. His strength may rival a storm's might but even the mightiest storm would not last forever.