Philip Strenger is an old soldier who fought for Temeria. However, when Nilfgaard invaded in the 3rd Northern War (The war happens during the events of The Witcher 3, he declared loyalty to the southern invaders. As a thanks, they granted him temporary control over the castle in “Crow’s Perch”, effectively giving him control over the entire region of Velen.
His men are seen terrorizing the countryside.
The Bloody Baron is a major character in an early storyline in the Witcher 3, where Geralt arrives at Crow’s Perch to find the Baron searching for his wife, Anna and child, Tamara. He agrees to help since the Baron has information about Ciri. The Witcher begins his search under the assumption that they have been kidnapped, though he quickly discovers that they might have left willingly.
Philip Strenger, it turns out, abused and beat his wife and daughter during his frequent drinking binges. While he had been at war, Anna had cheated on him, which made the Baron furious, and his temper had been terrible ever since.
To make matters worse, Anna turned pregnant at some point with the Baron’s child. Wanting no part of this, she and Anna decided to flee. Anna made a deal with the "Crones of Crookbag Bog" in order to get rid of the baby, making her carry out a forced abortion. This child, however, turns into a Botchling [check] and when the Baron learns of this, he immediately wants to rectify his actions. Geralt can here either help him kill it or turn it into a lubberkin, which can help him find the other missing family members.
During the story, Geralt spends a lot of time talking to the Baron, and while his actions are reprehensible, he claims he truly is sorry for what he did—however whether he will actually change his behavior is left up to the interpretation of the player.
The ultimate fate of the Baron and his family is dependent on the player’s actions during the entire Velen Questline.
The story behind his nickname is, ironically, not a bloody one. According to the innkeeper at The Crossroads in Velen, it happened during a raid in the war prior to the game, where they raided a paint factory, and spilled red paint down a river. The peasants thought it was running red with blood, and thus the nickname was born.
This is not to say that he hasn’t earned the moniker, though, as he is known as a ruthless and savage warrior and baron.