Could be a lot of things, you will have to work though and eliminate what it’s not to deep dive into what it is.
Could be you attracted something and the nightmare was a warning you were under attack. Could be you uncovered a guide and you are afraid of it so see it as scary, Could be an understanding about yourself your subconscious is trying to tell you, Could be a message from the angles of omnipotence.
Uiazel “initiates” you, so you could have been receiving energy for that and your mind interpreted it as scary. Or something else and maybe a combination of any and mundane and you were going to have a nightmare anyway.
This person online talked about their experience with Uiazel and I though it was interesting.
I see these as an opportunity to get to know myself better. I might not repeat the ritual unless I already planned to, but I would recontact the entity and ask about it.
Here is a collection of topics about dreamworking: