Joseph Dondarrion

=== Before the First Clash ===
Joseph started his life in the Brotherhood without Banners and is a descendant of the original Beric Dondarrion. When he came of age he took charge withing the Brotherhood and led a rebellion against House Tully alongside some minor houses in the Riverlands. The rebellion succeeded and King Bran allowed the Brotherhood to establish their own state at the cost of some territory, including Harrenhal. Joseph was named the General Kommissar of the Trident Union and ruled quite violently for time. Just before Bran's death, he managed to join the 7 kingdoms again in hopes of spreading his revolution further than the Riverlands.
=== During the First Clash ===
Joseph's violent rule became most prevolent when King Bran passed away and left the 7 kingdoms to war over his next position. There was a dispute between General Dondarrion and the Crusader Queen over Harrenhal which Joseph wanted back into the fold of the Riverlands. While being a follower of the Lord of Light and technically not even a lord the Crusader Queen stroke a deal with the General Kommissar. The Union could have Harrenhal but the Light of the Seven must be the dominant religion in the region.
The missionairies send by the Crownlands were soon found dead, hanging above the gates of Riverrun. The Crusader Queen summoned her fellow great council members and declared war on the Trident Union.
Together with only a single ally in the Westerlands, Joseph Dondarrion offered as best a resistance as he could against the entire country who fought against him. After losing to much territory and men, the General Kommissar surrendered and lost Harrenhal to the Crusader Queen. 
While the Trident Union licked their wounds there were already plans in place for a second war, this time way more prepared and trained for. The Kingdom of Dorne also made itself a prominent sponsor which could keep the war effort going for as long as the Union needed it. 
When everything was ready the suprise coronation of Jon Arryn the Second happened. This however didn't stop the war from happening and resulted in a bloody conflict between the Crusader Queen and the Kommissar. It was a final battle at Harrenhal were both the Queen and the Kommissar found their end. Joseph died at the end of a crusader's warhammer and was brutally beaten to death. 
While the both armies agreed to return to their homelands they would find their power usurped by others. Both Trident Union and Crusader Kingdom died out.
Joseph's Trident Union was overthrown by his family member Beric the Second and the previously overthrown House Tully. The Brotherhood went back to serving the Riverlands once again and the Trident Union seized to exist alongside it's violent ruler.
=== Legacy ===
The legacy of Joseph Dondarrion is somewhat debated, as he is remembered very fondly by the people of the riverlands themselves. This doesn't transfer to the lords and historians of westeros, who see him as a definite threat to their power and rule. His opponents usually point to the extremely bloody combat and ruthless rule during the first clash, while his supporters point to his stable regime in which the smallfolk reached an unparralelled level of equality.