It happened that night.
"These guards are so used to fighting monsters that they don't know how to spot an assassin," Apki thought as he put his back against the wall of the passageway and began to slide against it like a slug.
But then, maybe it was that Apki was simply that good of an assassin. He smiled at that because he knew he was not that good. His skills lay more in stealing than killing.
He had a sub-essence in the Dark Knight Order that made it possible for eyes to look past him without registering him. Once he hid himself well, the brains of others would simply tell them that he wasn't there because they did not expect an assassin to be there.
The problem was that if he met a paranoid guard who usually expected assassins in every shadow, then he would be detected. But this was the borderland. They fought monsters more than they thought about what an assassin does.