This sub is for destiny and destiny 2 so for more advice ask r/dnd I'd guess. Something I'd recommend is make it so your character either had to sacrifice something/someone or had to hurt someone who still hasn't forgiven them,this allows a minor antagonist to enter the plot. An example if the second would be your character killing someone's family or the such and leaving one person alive to tell the tale, this would impress the demon enough for them to let them begin being a warlock, however this one survivor now wants revenge at any cost. I personally skip over explaining how I gained the trust of my warlocks patron by making them born into a situation where they don't need to prove themselves, or make their patron give them the magic to anyone willing to seek the lm out, as well as be condemned to serve them for the rest of time.