Tensions were rising between houses Keller and Blackgard, and both parties were eager to go to war. The war eventually erupted when Auster Keller had the Maester of Blackgard killed. Lord Trovar Blackgard and his firstborn son, Trevas Blackgard, left Blackgard to go to battle, leaving behind Laina and a young Trevyr Blackgard and Sofina Blackgard.
The Blackgards were winning the war. However, during an unexpected ambush the Kellers managed to assassinate Trovar. Before Asterion could return to Blackgard to be named the new Lord of Ebonheart he was killed as well, by Tylan Keller himself. Two weeks after Asterion's death the Kellers came riding into Blackgard and had the entire city slums put to the torch. They then killed Laina and imprisoned Trevyr.
The Kellers had occupied Blackgard for almost seven years. During the time, Tylan Keller attempted to force the former allies of the Blackgards to become allies of House Keller by force. This included trying to force House Tully into an alliance by forcing Lord Ambrose Tully's daughter Addysen to marry him. When the Tullys refused, Auster Keller lead an attack on Riverrrun which inadvertently lead to the death of Addysen. An infuriated Ambrose would later pledge his support to the combined forces of Trevyr Blackgard and Lyonel Baratheon.
The Baratheons, a long-time ally of the Blackgards, were infuriated and demanded for Auster to retreat his forces from Blackgard. The Kellers refused and eventually rebelled against their liege. The war took a more dangerous turn when King Maekar Targaryen branded Auster Keller as a traitor to the crown.
Trevyr, now seventeen, decided that it was time to retake Blackgard and to destroy House Keller. Not by battle, but with sheer brilliance, cunning thinking and scheming. Earning him the nickname: Prowler. He managed to weaken House Keller by politically exploiting them and persuading several houses to stop supporting the Kellers. He also cut the shipment of food, armor and supplies to Blackgard. When House Keller was beginning to lose their grasp on Blackgard, Trevyr and a couple of other companions, killed Tylan in his sleep, leaving the Keller soldiers without anyone to lead them. The Kellers eventually left Blackgard and retreated back to Kell, their stronghold.
After the Kellers were expelled, Trevyr and Lyonel Baratheon marched with their troops towards the castle of Kell and besieged it. The battle of Kell didn't last more than a day and Trevyr was able to make it through Kell's great hall where he confronted Auster Keller in a deadly duel with Trevyr as the victor. While he was struggling to get on his feet, Trevyr's men tied Auster to his bed where they burned him alive. Trevyr ordered his troops to search the hall for Auster's family and eventually found Auster's wife with the last of her three children, hiding beneath the beds. Trevyr's men took the family in custody and brought them to King's Landing for the infamous Execution of the Kellers.
In the aftermath of Fourth Border Wars, the Kellers were no more. Trevyr was ruthless and cunning, and soon he defeated the Kellers and burned Kell to the ground. Trevyr put all surviving Keller to the sword, man, woman and child, to make an example of them for anyone who would dare challenge House Blackgard's dominance again. Trevyr subsequently had the corpses of the executed Kellers hung from the gates of Blackgard on public display. Trevyr later explained that he rather wanted the war to end peacefully. However, in order to make sure that the Blackgards would never face such a crisis again he had to inflict fear into people.
The extinction of House Keller marked the first major step in the return to glory of House Blackgard, in which Trevyr almost singlehandedly rebuilt the fortunes and strength of his House.
Trevyr's victory was immortalized by the minstrels in the popular song, Rainbow Rising.