Before the game
Blythe was a sellsword in the Fenns, alongside his brother. They were skilled at "guardin', watchin', (and) beatin'", and would do so for anyone with coin. The two had a close bond, both of them trusting the other to watch each other's backs. However, this profession earned them few friends and many enemies among the Fennfolk (who tend to hold grudges, as we're told). This included the Ratblades, a guild of a "bunch of petty crooks and scumbags" with an apparent reputation for taking their frustration out on hired help when things don't go their way. However, this life wasn't to last, as Blythe's brother was killed by a Ratblade. Blythe assumed that one of them stuck a dagger in his brother's back when he was walking home one night; enraged by this, he found the perpetrators and "saw that they got what was coming to them". But revenge couldn't fill the space left behind by his brother's absence.
Some time after this, Blythe went to pay respects at his brother's grave. However, he discovered that the grave was dug up, and the remains were nowhere to be found. The sellsword spent months searching; tracking the grave robbers down, he went north to the Arcane Commonwealth. There, he interrogated all of them, with methods that included breaking their arms, and all of them told him the same thing- a necromancer took the bodies they'd dug up. This was Gulretch, who was wanted for "all sorts of crimes against both the living and the dead." Gulretch fled to Amaranth Tower, but unintentionally leaves behind clues indicating this. Blythe assumed that he was trying to hide there, and entered the Mines with the intent of tracking the necromancer down and making him pay for taking his brother's body.