First some background:
I and four friends started playing D&D 5e. None of us has ever played D&D, including myself, but since the idea to give it a shot was mine, I volunteered to read the rules, listen to some podcast and be the DM.
As I'm new to this, I'll be running the Lost Mine of Phandelver campaign. First I did a session zero to create the characters and to define their backgrounds.
One of my players said he was going to be a Warlock Dark Elf. I asked him with whom he made a pact with. We went to the PHB to read the possibilities, ending to choose "Fiend". Inquiring him about the story of his character he said he was an high elf and as children he was playing with his 2 brothers when suddenly he blacked out and when he regained his senses he had his hands around his brother's neck. Scared, he ran away and later was contacted/touched by a mysterious entity (supposedly the Fiend) causing him to become a Dark Elf. Having episodes of berserking/rage/dark thoughts, he lived the rest of his childhood in the streets stealing and running from his brothers because he thinks they want revenge for what he did.
Not sure if subrace change is against the rules or the fact that he really didn't make the pact but I was so captivated by the story that I didn't contest anything.
Yet I want to explore more about his past and do some homebrew arc where some fiend princes or lords are trying to conquer the mortal plane through this character but I'm having difficulties finding lore or stories about Fiends.
So where can I find fiend lore on 5e or some campaign focused on fiends?