Black Walder Frey was the great grandson of Walder Frey and was the head of House Frey after emerging as the victory in the Bleeding of the Twins during the Second War of Conquest. Black Walder would become known as a kinslayer and become a dynastic stain like the rest of the Frey household and would ultimately be executed by his relatives following his defeated against Aegon VI Targaryen in the Second Battle of the Whispering Woods.
Black Walder emerged victorious during the Bleeding of the Twins, killing several of his family members in the process and claiming the Lordship of the Crossing upon the bodies of dozens and dozens of dead Freys. Black Walder would then enter the Second War of Conquest, fighting against Edmure Tully and Harry Stickland who were attempting to restore House Tully to Riverrun.
Walder would score many initial victories against the Tullys and the Golden Company, pushing them out of Riverrun and pursuing them through the Riverlands. It was not until Aegon VI Targaryen returned from his War in the Reach that aid was sent to the Riverlands. Aegon and Walder would meet in the fierce Second Battle of the Whispering Woods in which Aegon scored a decisive victory and captured Black Walder.
In exchange for a vow of fealty from the Twins and departing Riverrun, Black Walder was returned to House Frey and promptly executed on the basis of Kinslaying.