New DM need help setting up a dream

Hi everyone. 

I’m a new DM playing with a group of ppl who are also new to DND. 
We’re playing the starter set Heroes of the Borderland. 

My players being new, they haven’t really created characters with a lot of depth. 
And so I would like to create a dream for one of them in order to add some layers to the narrative. 

The PC is an elf wizard with a background as a Guide. 
In the little backstory that I asked them to write, this character comes from a very strict family and has some trauma built in. 
They recently came across an idol (in honor of Baphomet) that Cultists were trying to activate. And now they’re going to have their first long rest. 

During this rest I wanted the elf to have a dream. 
The broad lines of it would be memory of this character as a child running into their parents (who were also wizards) engaged in a ritual to summon a demon. It would be blended with the idol they came across

My question is: what kind of demon do you think would be relevant for this dream to awaken a fear into this PC? 

Would you have any other advice? 

thanks for the help. 

From how you wrote this, it looks like you are adding an element of "your parents summoned demons" that did not previously exist in the player's backstory.  The first question you need to ask yourself is if you, as the DM, have permission to do this.  Most of D&D is DM-driven, with backstories being one of the player-driven aspects of the game.  DMs are (and often should) work with the backstory to improve and enhance their narrative, but it is the player's backstory that is driving the DM's decision and not the DM writing new elements of the backstory in a vacuum.  

Permission can come in many forms.  It can be explicit ("do what you want with my backstory, I don't want to write one"; "I left these elements vague so you can fill them in") or implied ("My character has no memory of this part of their life"; a backstory that seems to leave something vague for the DM even if not outright said), but still needs to exist.  The facts here do not support that - you have a character whose only backstory stated is "I have strict parents and trauma history" and nothing to suggest demon summoning was involved (as presented here).  So, you are not only adding something that does not seem to exist - you are adding something to the only part of their backstory they cared to give you, and thus could be drastically interfering with the narrative they want to tell.

That does not mean you cannot do this, but it does mean you need extra careful. Since you have a group that is new and generally failed to fill out backstory, you could have a conversation with them - “hey, I want to work in more personal stories based on backstories, can you all give me a little more detail or would you like me to flesh them out on my own?”

Moving on to your question itself, if I were in your shoes, I would slightly pivot and have the parents summon a Devil, not a Demon.  In D&D Lore, devils are the lawful evil side of fiends, while Demons are more chaotic.  Given the parents in this case are strict, it might make more sense for them to be summoning a lawful entity than a chaotic one.  Given Devils and Demons are historically at war, a Devil antagonistic to Baphomet would put the party in a position where they might have to make a literal deal with the devil to get an advantage in whatever they are currently investigating.  That kind of choice - “this might help, but it also could bite us later, maybe we just kill the devil now?” leads to some interesting roleplay choices.