So essentially;

Japanese (land of reeds) painter ends up shipwrecked and washed in the lands between (Ireland). enjoys painting the erdtree has a sentient cursed blade. crucible knight arrives to confront painter painter offers sword and befriends crucible knight cannot kill painter knight vows to protect painter from other crucible knights. knight fails to do one job. out of anger kills his brethren, commits heresy in the process and stabs himself with the cursed blade. combination of killing self, causes primordial energy caused by the crucible, which itself is basically what happens when you let dragons breed with humans, fuses the dead painter, the spirit of the blade and the crucible knight together. this explains why the armor is permanently attached to executors skull, why he is skilled with a katana and its alternate move set (because of the sentient sword), and why the executor can literally become one of the more powerful forms of the aspect of the crucible.

It’s fucking neat, and executor is pretty much a fear and hunger marriage in a fromsoft game.