In Cala Bryn Sturgis, Roland’s group encounters Father Callahan, a character from Stephen King’s novel 'Salem’s Lot.
Father Donald Callahan first appears as a Catholic priest in 'Salem’s Lot, King’s 1975 vampire novel about the fictional Maine town of Jerusalem’s Lot. His faith is badly shaken when he confronts the vampire Kurt Barlow and is unable to defeat him. That failure becomes a defining part of his later story.
Callahan returns in The Dark Tower series, especially The Dark Tower V: Wolves of the Calla. By then, he has reached Mid-World and is serving as the priest of Calla Bryn Sturgis. He tells Roland’s ka-tet how he escaped 'Salem’s Lot and eventually arrived in the Calla, linking the otherwise separate horror novel to King’s larger multiverse.
A common mix-up is confusing Father Callahan with characters from It or The Shining. Those books also contain famous King characters, but neither Bill Denbrough nor Jack Torrance is the priest encountered in Calla Bryn Sturgis.