Victorion is the "daughter" of Solus and Nexus Prime. I haven't really thought out the details behind that, except that she wasn't forged as a combiner, but due to Nexus's part in her formation she had the capability to become one. Solus and Nexus prepared for this eventuality, with Solus producing a great combiner-sized sword, gauntlets and boots for her to use in combined form. Victorion fought alongside Solus and Nexus during the war of the Primes, but was fatally wounded, along with Solus, by Galvatron. Nexus couldn't save Solus, but was able to transplant Victorion's spark into the sword Solus had forged for her. Knowing that Galvatron would most likely not be defeated, he decided to leave her spark there so that she could remain safe for the remainder of the war, after which he planned to use the Enigma to donate her spark to a combined form of some of his loyalists. This tampering also allowed the sword itself to split into separate weapons and recombine. While Nexus himself was obviously defeated, followers of him and Solus managed to retain and preserve the sword, over time becoming a small and distinguished order that served under the Mistress of Flame, the Torchbearers.
It would not be until millions of years later that the current incarnation of the Torchbearers, still possessing the components of Victorion's blade, would come into contact with the Enigma and reconstitute the daughter of the Prime they revere. Victorion herself is very much an anachronism, out of step with modern cybertronian culture and having to adapt quickly to the current political climate. Predisposed to trust the Mistress of Flame, she follows her lead to further looks to Optimus as leader, since he is the last true Prime. However, the Mistress does not feel completely comfortable having the daughter of Solus around, fearing her as a potential threat to her power on Caminus.