She defeats the Dark Cloud and absorbs the Vampire KIng vampire essence, unfortunately returning her to being a vampire and being immortal.
After returning to being the “vampire queen”, Marceline comes to terms with her immortality and her vampirism.
Throughout the entire series she struggles with her morality, emotional stability, and immortality, it’s shown multiple times she even says as such in the season one, but in this specific arc of the show is where I think she grows the most both literally while being mortal for a little while and metaphorically in her emotions and ability to accept things and “grow up” and have more empathy.
Bonnibelle and marceline's relationship also grows a bit and they become closer, again I think this is where their relationship grows the most except for maybe the s3e10 “what was missing” and s5e29 “sky witch”.
Bonnie offers a room in the castle of the candy kingdom for her to stay in but she declines.