Try 666
Try random seeds and look for crosses
Try 12345

I feel if you want to make a seed scary you should fine texture packs online and find mobs that are scray and make some.

There are no "special" or "haunted" seeds. If you want to make a video like that, you should download a mod that adds scary things.

Uhh, Im not familiar with scary seeds. I guess you could find some really weird and unnatural features in a seed like a weird cliff or broken up mountain. There are no special seeds with certain entities either. They're all the same like find three wither skulls to create the Wither, or find the End Stronghold to go to the End Dragon.

Things that can make your world "scary" is finding texture packs online or on this website and download them and put them into your Resource Packs folder in your minecraft folder.