I have been looking through the list of deities in the 5e Player’s Handbook, as well as some of the big nasties in Mordenkainen’s Tome of Foes, but there don’t appear to be any deities that are associated specifically and primarily with fear.
Tharizdun is associated with darkness and madness, and Demogorgon is pure gibbering madness; indeed, all of the Demon Princess are obviously terrifying to behold. Star Spawn and Elder Evils are similar, in that they are undeniably terrifying, but they aren’t driven by the desire to cause fear. For all of these creatures, fear is a natural secondary response to the other horrors that they want to bring to pass. It isn’t their driving force or domain.
I’m looking for something that is primarily associated with pure fear. Does such a thing exist in any edition of D&D? Any setting is fine, since this is for a Planescape campaign. I’m perfectly happy to homebrew my own fear-based deity, but if something already exists, I’d love to be able to make use of it.