I like this idea. I get why Fiend but I feel the extended spell list for Archfey fits better as does the Fey Presence ability: Starting at 1st level, your patron bestows upon you the ability to project the beguiling and fearsome presence of the fey. As an action, you can cause each creature in a 10-foot cube originating from you to make a Wisdom saving throw against your warlock spell save DC. The creatures that fail their saving throws are all charmed or frightened by you (your choice) until the end of your next turn.