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.