Prayers to Broken Stone

Driven mad by dragon sickness, Thorin banishes the company and locks himself inside Erebor. Days pass, and fearing for Thorin's safety, Bilbo descends into Erebor to find him. Instead, Bilbo discovers Thorin slowly becoming a dragon himself, and must do what he can to save him before Thorin succumbs to a curse that twists his body and mind. (Thorin/Bilbo, BoFA AU, novel-length)