Okay guys I've been thinking quite for a long time.
I read a danmachi story were only women can have a falna called 'Men can't be adventurers so I'll be one anyway' by asianwithagun.
His story is very good and an idea has popped up in my mind. This is a potential story so I will layout a few details. If men can't be augmented by the Gods then they will turn to the spirits and focus on how to perfect it so that it can be more suitable when combating against monsters.
Lore
King Arthur and his knights sought the help of the lady spirit of the lake and other spirits to fight off against creatures from the spirit realm that were attacking Camelot as their weapons and might were ineffective. They created a perfect synchronization of a mortal soul and a spirit.
This made King Arthur the first vessel and founder of the Vessel knights. With their combined might, they were victorius and banished the creatures back to were they came. The creatures they fought would soon be called demons of the void.
As the spirits that aided them descibed the void to be the Spirit Realm's darkest region were major and minor spirits avoid traveling. The Spirits stayed at their side to protect Camelot.
Everything remained peaceful until the betrayal of Mordred and his followers. This sparks up another war that brought the downfall of Avalon and the death of Arthur and his traitorous and illagitamte son.
The surviving knights of Avalon fled from the fallen kingdom and went into hiding. The fates of Mordred's follower remained unknown.
The Vessel knights reformed to the Vessel Assasins and fled to secrecy with a few other people who knew of their existance now living with the Creed. As they not only focus on monsters, but criminals and tyrants as well.
Percival, Lancelot, Morien, and Tristan went to their separate ways and founded four houses were they would train future generations of Ethereal Assassins.