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Spoiler
The butterflies feed on her blood, which means she is essentially going to die early supporting them this way. When their master becomes too weak to continue this, the butterflies devour their master too. She was willing to die for her cause, to be free, and took this gamble when she raised them. Roxana knew this would be how she would die if she survived her family. She's not like the other character in the story who can control beasts on command. So, she is essentially dying slowly because of them. Cassis purifies her to save her life and to extend her life.