but not a pagan from Iceland which shouldn't be able to know that an empire called Byzantium exists. It's just a matter of distance and common sense.
Actually Norse pagans
definitely know that Miklagard exists, because they're sending their kids there to serve in the Varangian Guard (which historically formed in the 9th century, but in game depends on what the Emperor is doing...)
Since the beginning of the game with Byzantium i'm playing im on 99% threat and there is no way to lower it.
... except there
is a way to lower it. You can just 'not be at war for a little while' (you can knock a lot of threat off by granting independence to part of your realm, but you probably don't want to do that, given that you're expanding).
You're describing 'being at total war since the beginning of the game'. So obviously your threat is only going to build - as far as the other nations are concerned, the Roman Empire is resurgent, it's on its way to reclaiming all of its old borders (otherwise known as 'almost all of Europe and North Africa') and it could easily be coming for them next.
And - by the look of that map - the Emperor isn't interested in peace, he's not making non-aggression pacts or alliances with anyone. He's just picking a target and crushing it, then picking his next target and crushing that, and so on.
It would be absurd for nations & rulers
not to be frightened of that, and try to do something about it.
If you saw an AI behaving like that,
you would start preparing yourself.