Word is Acceptance: Muse is Mordred
MORDRED - KNIGHT OF TREACHERY REBELLION. Send me hc + a word of your choosing and I’ll write a headcanon relating to that word
ACCEPTANCE — a word that he yearned to understand. He'd wanted nothing more than to be accepted by King Arthur, the one meant to be his father. To not have to hide who he was. Part of him desperately wanted Arthur to accept who he was, not wanting to go through with his rebellion or betray the King. His mother, Morgana, raised him as best as she could given the circumstances — but she was always truthful about the fact that he would never be accepted by Arthur. He always knew she was right, mother knows best after all, but he wanted to believe there was a world where she was wrong and they were living in it.
Unfortunately, they weren't — once enough was enough, long after already being made a member of the Round Table, Mordred had taken off his helmet to reveal his true identity in private with Arthur. He'd hoped that meeting with Arthur alone would make things less intense and not put Arthur on the spot, but it seemed to do the opposite — it invoked Arthur's rage, and a barrage of insults were hurled at Mordred. The one that hurt him the most was that he would never be seen as a son, nor would he ever be fit to rule Camelot.
Everyone knows how that story ends from there.
That being said, this is an insecurity that is taken with Mordred after-death.
When he began a romantic relationship with Vivianne, the woman who summoned him, this became something he worried about internally for a fairly long period before actually speaking to Vivianne directly about it. His reputation isn't the best to put it lightly, and she was well aware about the legends of King Arthur and especially aware of the Knight of Treachery's actions. He'd always been so confused why she seemed happily content having summoned him, even more confused at the fact that he seemed to be the exact Heroic Spirit she was targeting for her summons.
At first, he thought it was because she wanted someone she knew would use underhanded tactics to win a fight — but that wasn't it. Then he figured it was a fluke that it was him that was summoned, the catalyst that was used was one that had a chance to call forth any of the other Knights of the Round after all — but that didn't seem to be the case either.
To this day, he still doesn't understand why she was so openly willing to accept him, even after finding out the complications about his gender and the dragon blood that coursed his veins. She didn't bat an eye at either of them, if anything, it only made her accept him even more.
Acceptance was something he once thought himself undeserving of, something he thought he needed his father's recognition for first before anything else. But when he met Viv, everything changed; it was like the world flipped itself upside down for him. Expecting to be used as a tool, only to be embraced as a person.
There are many things he loves about Vivianne — many things that are impossible for him to put into the right words to describe. But the thing that made him fall for her was the simplistic feeling of being seen by her.
It's still something he struggles with — the feeling of being accepted, the fear of being denied. Sometimes he wonders if one day, Vivianne is going to realize she doesn't need him around — but he's well aware that day won't happen, even if he's scared of the possibility.
He tends to mumble in his sleep, and there have been nights where Vivianne has woken him up from a bad dream of his where this very fear has come up — but she has always perfectly reassured him. Acceptance was no longer something he had to fight for, nor was it something he needed to earn; it was given to him without any question.
Viv was the one who summoned him, but he was the one who got lucky.