Catelyn Tully was not having a good time in the North, she was a widow and didn't have any children. Over the years she watched Ashara Dayne give birth to Robb, then Dyanna (Or previously Sansa) and Arya.
She was bitter about how her life turned out. She tried multiple times to get Ned Starks attention but failed every time. Also, she felt people of the North did not respect her enough.
She told King Stark to build a sept, these are places of worship in the Faith of the Seven. They are traditionally decorated with seven-pointed stars and artistic depictions of the Seven, such as statues or sculptures.
But Ned sighed and explained, "I can't do this Lady Catelyn, since creating a sept in the middle of Winterfell will cause unrest among my vassals and people."
Ned saw Catlelyn frown, but he calmly continued"You must understand North has a very strained relationship with Faith of Seven, who has called us savages and demon worshipers because we worship the Valar's."
Catelyn frowned and said"Then thank you for explaining." with that she walked out of the throne room.
Ned sighed and said, "She is getting more and more out of control as time goes on."
Ashara, who was by his side nodded and replied"She has been trying to get on your good side for a while now."
Ashara felt Catelyn had a subtlety of a bull.But then Ned frowned and said, "So you noticed that too."
Sahara nodded and replied, "Yes, my guess is Hoster Tully is pushing her, he is probably hoping you will notice her and make her your second wife."
Ned scowled upon hearing this, he had a very beautiful family with Ashara and he was not about to destroy it for some foolish woman.Ned only left her be for so long, since she was Brandon's wife.
Ned then questioned, "Did she do anything else?"
Ashara groaned replied"Yes, I heard she has been trying to get the servants to turn to Faith of Seven for a while now."
Ned sighed, while Ashara added"She has been advising me to hire a septa.
Septas are the female clergy of the Faith of the Seven. They are women sworn to celibacy, sometimes serving noble houses as governesses and tutors to the daughters of lords, teaching them in matters of etiquette and history and activities such as sewing.
Neither Ashara nor Ned wanted their children to grow under the influence of a septa, both of them were planning to teach their children ways of North and Valar, Ashara was planning on teaching her not only politics, but also involving her brother to teach there children to be a swordsman.
The house of Dayne had traditions like the wielder of Dawn is always given the title of Sword of the Morning, and only a knight of House Dayne who is deemed worthy can carry it.
Also, her Arthur Dayne was now in Middle Earth guarding Lyana and her son Anduin II Stark(This is Jon Snow).
Then came the Greyjoy rebellion.