Trying to come up with a word - native English speakers please help?

Hello everyone,

I'm currently writing a paper in English and have been trying to come up with a word for a long time already. I'm not an English native myself, so I was wondering if someone here could help me out.

I'm looking for a word that describes "fear" (an adjective). It should mean something like "hard to define/pinpont", something "unclear and/or mysterious"

The sentence goes something like: "His obsession is caused by a xxx fear."

What I mean to say is that one can't exactly say where the fear comes from or what this person actually fears, it's just some very unclear but in a way strong (perhaps even creepy) fear that comes from deep within that person.

Can I use "elusive"? Like "His obsession is caused by a subtle and elusive fear that stems from deep within him."?

Or what other words are there?

Thanks so much!