Luna didn't have many friends. There were the bearers of the Elements of Harmony, of course, but not much more than that. Then she met a pony named Altered Morph. Luna had gotten to know her very well. And then she changed. Forgot everything. And she had to say goodbye to the pony she'd grow to care about. Then it it started all over again.
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What just happened? Is there more to this?
It sounds like Luna met a pony whose very core nature is in flux that she's come to enjoy. She constantly changes personality and form with no memories. Luna is trying to figure out why its happening and stop it. At this point, she hasn't found out how. :(
To clear things up, Altered Morph's cutie mark constantly changes, and when it does, she forgets everything that happens previous. Luna is trying to figure out whats going on, because some cutie marks make her do bad, even evil things.