To explain everything, the prequel episode of The 100 showed what happened after Becca returned to Earth two years after the apocalypse revealing the origin of the Grounders story.
It’s been known for quite some time that Becca created Nightblood as a way to defend the human nervous system from deadly radiation. Considering the blood was black but it is later revealed that it was Callie’s idea to use the name in The 100.
The First Keeper of the Flames
After receiving a blood alteration with Becca’s serum, Callie sought to give it to thousands more people in The 100. And she needed to find someone to replace Becca; whether she was herself or not remains to be seen.
It is presumed that she became the second commander after Becca, and thus, Callie surprisingly became the first Keeper of the Flame in the history of The 100.
The final code
Bill Cadogan and the Disciples have been behind the final code for hundreds of years in The 100, and it turns out that Becca told Callie what it was shortly before she died being something that perhaps only the Keepers of the Flame knew.
The Latin phrase for “goodbye for now”. Block the Flame until someone new can be found to join it so they can save the human race and not allow the Disciples of the Second Dawn to use their technology in The 100.