DISCLAIMER: I do not own Rikki Chadwick as she's from the H2O fandom. Nor do I own HP or Marvel this is purely for for enjoyment. However the fanfic is my own, so please show respect not post on other sites (I already have it on AO3/Wattpad).
Life as a mermaid was everything to Rikki. To the others that had been changed on that hot full moon night on Mako, their lives evolved and moved on. They got jobs and families that took them further from the sea; but that was never something that she could do. So as Emma, Cleo and Bella aged alongside their loved ones, she did not. Her healing abilities as a mer of the seas kept her looking like she was no more than twenty five, even as her friends passed on.
Rikki would make several different identities over the years, occasionally checking in on the families of her old friends. She would visit the different pods of natural born mermaids throughout the different oceans occasionally meeting a mer like her who had changed beneath a full moon. However, she would always find herself back upon Mako.
One such night as she was just about to reach her second century of existence Mako's full moon answered her silent wish as she dosed in the waters of the moon pool.
Life as the Master of Death weighed heavily upon the shoulders of Harry Potter. Infinity was far too long. Death had given the choice long ago of having a life mate, but Harry could never subject Ginny to the heavy responsibilities that came with immortality. So instead one by one he saw his loved one's die, while he did not.
The Wizarding World died soon afterward the loss of so much magical life devastated the ley lines, causing a domino effect throughout the world. The magical beings went into hiding like in the dark ages and the few of magical-birth that were left disappeared shortly after the Snap.
Death had offered an alternative in the end, Harry could go back to the start and stop the first war. Even without the Hallows in his hands yet, as a toddler if he could get the loyalty of an impartial, he would be protected by Death's magic until the Hallows could be attained. However, Harry didn't want it. He knew that he couldn't look upon both sides objectively, even now after being the Master of Death for two hundred years he still felt a pull toward the Light.
Harry never wished for forever, he never wanted it.
Death seeing the pain his master held gave him an ancient spell that could only be enacted once, it would seek out another in a different dimension to complete their goal and who would trade lives with his master.