Homebrew BBEG advice needed

I’d like to start off by saying this is the first campaign I’ll actually dm, but I’ve helped some other dm friends of mine come up with ideas and write their campaigns.

I’m currently working on a homebrew psychological horror campaign and found an idea for a BBEG that I absolutely adore, however, the creature I decided to go with was another homebrew idea that doesn’t have any set stats.

While I was doing my research for good BBEG candidates, I stumbled upon a miniature called the ‘False Seraph’. After reading a bit more about its lore, I decided it’d be the perfect fit for my campaign, but there aren’t any stats for it or even suggested stats that I could find.

Normally, I would take this as an opportunity to make this being into whatever I want it to be, but I don’t want to set my players up for failure or make it too easy for them to beat.

I want this creature to be an all seeing eye of despair. He watches over a lost isle, permanently changing the residents. Each town has been corrupted into different cult like sects, giving power to The Watcher/Seer via sacrificial rituals of those who break from routine or begin to question. The longer you’re on the island, the foggier your memory becomes, making you more susceptible to either join a sect, or become a sacrifice. After awhile, the whole island becomes like a labyrinth, making it nearly impossible to find the one village that can help free the island from The Watcher/Seer.

What stats/spells/immunities/etc. do you think sound like they’d be challenging, but doable for your average player? I really want to play into the creepy unknown false god trickster spirit energy, but Im struggling with making sure he’s not too overpowered or too easy to kill.