Hamato Lore
After Oroku Saki, Karai inherited the dark armor and became the shredder of her generation. The armor continued to be passed down the family and the Hamato Clan became highly respected/feared, until Splinter.
Since he was born, Splinter has been guided by his mother, Atsuko, and his Grandpa Sho to continue the family legacy. Atsuko believed in the existence of Yokai from ancient family texts about them (specifically the Miyamoto lineage) coming after the Hamato family over generations. Her goal was to purge the Yokai from existence, believing them to be entities that would bring her family harm.
Atsuko finds the whereabouts of the Edo City but gives birth to twins, Miwa and Yoshi, before she could take action. Atsuko rejected Miwa and cast her out of the family because she was ill, so Splinter became the heir (there will be a separate post about Miwa's history).
Atsuko dies early in Splinter's life. Grandpa Sho tells Splinter about Atsuko’s plans to destroy Yokai kind and encourages him to finish what she started, so when Splinter’s finally old enough to use the Dark Armor, he’ll destroy Japan's hidden city, Neo Edo.
Splinter was unsure about the Hamato Clan’s beliefs and decisions but when Atsuko dies, he feels like he has no choice but to embrace it. Splinter only realizes how much damage he does in Neo Edo when Miyamoto Usagi fights back and sheds the light for him. Splinter realizes belatedly that he made a mistake, so he abandons the armor, which allows Miyamoto to break it apart and spread it across the world. Splinter leaves his home and moves to New York, thus ending the Hamato Clan rule in Japan.