I need help with a characters phobia

I have a character who has a fear of the dark in his adulthood. I wanna have an interesting backstory as to why other than he just never got over from his childhood. I want it to start from his childhood but something traumatic enough to make him afraid of the dark.

What is his childhood like?

So in his adulthood, he's a paranormal investigator. But it was only until his late 20's where he was really introduced to the supernatural.

Any specific dark related monsters?
(Fyi I am going to bed soon cuz it 11:32 PM in my timezone)

Intriguing!!!

I could make one up that likes to terrify children. Or one that creates traumatic realistic nightmares.

Again intriguing!!

So what absolutely has to happen in this kids childhood that maybe you can draw out of??

Well, there is an actual urban theory about shadow monsters. Usually, people see animals that scare them the most but pure black with red eyes. My mum and I have seen one. My mum saw a hound with glowing red eyes. I saw 3 giant centipedes.

I haven't actually thought of his childhood until now.

Then draw from that first. Think about what has to happen and what could. I mean if you want someone to have a lifelong fear of the dark then maybe they encountered something scary in the dark.