So with three books claiming keyness for Knull I figured it’s best to lay out the events of Thor God of Thunder #6, Venom #3, and Venom #4 to clear up any confusion.
In Thor God of Thunder #6 we get a few panels throughout a few pages of these masked warriors that have killed each other or so we thought. The character in the black armor would years later go on to be known as Knull.
In Venom #3 Miles and Venom defeat the symbiote dragon known as Grendel and from his symbiote the avatar of Knull rises and is in the majority of the book as the avatar of Knull except the end. In the last page we get the actual Knull not his avatar in one panel on the throne.
In Venom #4 we get Knull creation and origin as well as the origin of the Necrosword this is no avatar appearance. We also get the explanation of why Knull was in Thor God of Thunder #6. This book flips between actual Knull and the current avatar of Knull that was seen in Venom #3. At the end of Venom #4 the avatar of Knull that was throughout Venom #3 withers away and turns back into Grendel the symbiote dragon while capturing Miles in its tendrils and flying away.
So there you have it the beginning of Knull in all its glory. I think all 3 books are important for their own reasons and one isn’t better than the other. People will choose their favorite and try and discredit the other books but it’s always best when in doubt of a 1st own all the books that way you don’t have to choose sides. Full disclaimer I own all 3 books some in multiples and variants.