Caius, in his original life, was a l'Cie, tasked with the duty to protect Yuel. Yuel too, is a servant of Etro, who granted her the power to look into the future. Serah also develops the same power as Yuel in the game. Caius had to defeat the previous guardian of Yuel to recieve this task. Noel would have been the next if he didn't pussy out and actually killed him, instead of Caius killing himself.
Since Caius lived so long because of his focus to protect the continueously reincarted Yuel and all the feedback from Yuel about the future, he's the one who can see the proper timeline and fix paradoxes. However since looking into the future shortens the lifespan of the person doing it, Yuels didn't live long, all died at the age of 15. Caius wanted this curse to end for Yuel so they could live their life and this was why he set this plan in motion. Yuel's "tragedy" (Caius' words) started when Cocoon didn't actually fall, but was put on a pillar.
However Noel's Yuel (his timeline) didn't mind the reincarnation (it's the one who dies in Academia with a smile on her face, saying something like "we will meet again"). Noel tried to explain this to Caius at the end of the game, and he almost listened but went ahead with his plan and he killed himself instead because he saw no other way out to free Yuel. Every other outcome for the future without kiliing Etro would have meant Yuel's suffering continued, that's how Caius saw it anyway. So yeah, he did pretty much defy Yuel's and his fate as a l'Cie, by going for the one thing Etro didn't anticipate? But the original cast defying their fate is what set Caius' plan in motion, so the timeline was fixed to what it was supposed to be in the first place.
As for his sword being called Ragnarok, don't remember any of that. The only name I remember for it was "Soul Edge" ;p