Welcome to the Wild West, circa 1876. You are Jericho Cross, seasoned outlaw.
Looking for a big score, you raid a secret treasure train. Unknown to you, this train belongs to the Darkwatch, and ancient organization committed to defeating supernatural evils. The cargo of this train is not money or jewels, but a captured vampire lord being transported to the Darkwatch Citadel for interrogation.
You blow open what you think is the treasure vault on the train, only to release the oldest and most powerful creature of his kind. Perhaps from perverse gratitude, this vampire doesn't kill you. He just bites you, so you slowly turn into a vampire yourself!
With growing supernatural powers, you hunt Lazarus, the vampire who sired you. If you gun him down before the Curse overcomes you completely, you might win back your soul. If you fail, you will be damned to walk the earth as an undead monster, having condemned the West to despair under the rule of evil incarnate.