So Lucifer was an archangel, but he got sent to Hell. When he did, he turned into this great value cult of the lamb looking ass king and yeah. He had two right hand men (kinda like seraphim to God) that served him directly. Those two were decided a century before the deaths of the current ones (how? Spidey sense). The two that had served him for almost 10 thousand years eventually died because they got exorcised during the Great Exorcism of 1953-1957 (Otis was born in 1910, while Hunter was born in 1913, so it was a shocker when the previous right hands died earlier than prophesized). Lucifer chose the two's destinies of sin (cus he can do that, apparently) and boom, new henchmen, pretty much. Hunter strayed from the path he was on of sin and evil, actually making friends and learning to care about people, so naturally, Lucifer was pissed. He tried to kill Hunter, who narrowly avoided it and ran away, hiding from him. Lucifer tried finding another right hand man, but instead, found a lover on Earth (oh golly). Soon, Karma was born (imagine banging a literal god that must've been either epic or epically painful). His plan was to drive Karma to a life of sin and become his next right hand to replace Hunter, but during the second Great Exorcism of 2010-2013 (which the previous right hands were expected to die), the casualties were so bad, Lucifer had to ban the Spirit Portals (the way demons could enter Earth(I'll get into that later)), so yeah, Lucifer went to get the milk and Karma was a good person so he was on Lucifer's hit list at that point.

(Trying to decide wether to canonically ship Otis and Lucifer or not...)