The Tully's are followers of the Seven Gods, all though it is rumored that they worship a secret God of the Rivers and that they seal all of their agreements with Blood-Oaths. It is well known that house Tully made quite a big number of enemies inside the realm, among which are counted the Lannisters and the Belmores.
Riverking Cosgrove Tully
After more than 300 years since Edmyn the Sourfish passed, young Cosgrove Tully found interest in his ancestors and even still at a young age, the mystery behind the Sourfish eludes him. Born and raised at the Dragonstone, Cosgrove never saw the beauty of Riverrun.
Living with a crown upon his head for as long as he remembers, Cosgrove never felt the urge to explore the unknown Essos, to subjugate the Westerosi kings, to find the Dragons or to ride the North. His only wish was to see Riverrun and find the secret book of "Family, Duty, Honor" and unlock it's secrets. But with the Castle in Ruins and the realm in chaos, the task is hard for a young king and daggers of his ancestral foes sharpen at every corner.
"Such a young and foolish boy. He sees himself rulling from the top of the Trident. He dreams of the Twins, he dreams of the Riverrun. He never saw the Winter, he was born in safety with a crown on his head. Let us hope that with age comes wisdom, because if he pursues this dream, he pursues his death." - Maester Ormond's diary.