Merynis

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She was originally conceived when some sort of sex/affection-related demon had sexual relations, in secret, with a female fairy who was fairly high-up in Neil's chain of command. This fairy was also the mother of Orion, who became Merynis' half-brother and good friend. Neil exiled the female fairy for directly disobeying orders that he had made, notably being vows of chastity for his high-rankings fairies, and to not cohort with demons, as demons had recently been directed as attack forces to Neil's planet in the Separ Dimension. Despite Merynis being the offspring of a demon, she was still part fairy, so Neil treated her as such. She was, however, quite a rebellious person at times. And, though she did not know it, she had the ability to gain energy from love. For this reason, Neil wanted Merynis not to do stuff due to the danger of her killing anyone who she had intercourse with, and the reason he knew this was because her father had those traits as well.

However, Merynis' curiosity, mixed with her rebellious personality, caused her to disobey Neil, and she then ventured out into the Separ. She was inexplicably drawn to Hell, where she fell in love with a demon. She would often go out in secret to visit this demon, but as of yet she did not have intercourse with him. Neil soon found out about this and told her not to do this, but she was defiant, and on that day she had intercourse with him, and a child was conceived. The demon, though, did become much weaker during the process, and realized that she was the cause of it, and became afraid of her. After this, he often tried to push away from the relationship, because all along he had only cared for having sexual relations with her, and not romantic relations.

So, a period of time later, Merynis had the child (Which was a nonbinary being), and named it Enyredies. Neil became angered over this, and punished Merynis for disobeying him. He then sent Enyredies off to Hell to his father, where he then simply abandoned the kid. Enyredies was later taken in by Asmodeus.

Merynis continued to be rebellious in this time. She tried to escape, but was stopped by Orion. Orion really only wanted the best for her and he liked her as a half-sister, and didn't want her to go, but Merynis didn't want to listen to him or Neil. Orion was still trying to stop her and being persistent about it, so she then wounds him in anger. Neil then notices this and becomes angered towards her for doing something this horrible. Then he uses magic to put her under a Clay Curse, making her body constantly change shape and form beyond her control, effectively keeping her in place as an ever-changing blob.

However, the Clay Curse wasn't meant to be used on Fairies, only on demons. Neil hadn't accounted for this. And, eventually, Merynis' qualities not only made her free of the Clay Curse, but she also gained from it, now having the ability to change her form at will, which she soon discovered. With this knowledge, she left Neil's realm, and even left the Separ, not wanting to be there anymore. She arrived on Ludus then because plot. Let's say she found an altar that goes from a Separ planet to Ludus.

Neil found out about this as well. So, he made it so that her power would eventually drain, and that she would die, unless she came back to his realm. So, she had the option to just go back to his realm, but she didn't because she's just like that. She knew that she could take in energy from love, and was lucky enough to find some honeymooners nearby.

Later, she found the New Trolland Labs, and was employed by Lord Trollus I. Thinking that her abilities were outstanding, the Trolland Labs took some of her DNA and mixed it with the DNA of the Trollers to make the Changeling army that appears in TDE:U. This Changeling army was made to age more rapidly than normal things, solely for the purpose of having them be old enough to participate in battles as soon as possible. But this rapid aging also led to the downfall of the army because then they started getting too old and died out. Which is why things happened.

After Lord Trollus I died, his son, Lord Trollus II, took over. His son kept a watchful eye over Merynis because of her relation to his father. When she died and her daughter was just orphaned, he took her in. Crescent was also later betrothed to Lord Trollus III, the son of Lord Trollus II. The New New Trolland Labs also took some of Crescent's DNA and mixed it with Trollers to create some more powerful Changelings, and this time they made them not age really quickly.

Later, Crescent took the remains of the Troller race and the Changeling army to Equestria to take it over and make as their home, which was easy to do because all the important people who should have been there were not there, though she was later stopped by Starbreeze, much to the disdain of Merynis.

Neil and Orion have special abilities that allow them to retain some memories of alternate timeline versions of themselves. In the Fixed Timeline, they had memories of Merynis escaping and doing bad things so they took proper measures to stop the bad things from ever having had happened.