I was skeptical about them introducing Knull when he’s way too cosmic-level for our protagonists, but in the end I think Kelly Marcel and Tom Hardy did a good job at keeping things reasonable; The solution is not defeating Knull himself, just keeping him from being released. But with his character being established, it gets that introduction and setup out of the way for future films to build up to proper power-scaling in fighting Knull, if we get more films.
So yeah, Knull felt tacked on and that’s probably for the better. And it really makes me wonder if he was some begrudging executive mandate because tbf…
You could easily imagine a version of the script without Knull. The Xenophages are just Symbiote predators who have chased their prey across the galaxy, and that’s why Riot selected Earth as a safe haven. In the comics, they find Symbiotes with a longer bond with their host to be especially tasty, so instead of the codex being the result of Symbrock, it’s just them being a desirable meal. A bond like this (specifically a resurrection) creates a chemical that Xenophages can smell.
This is why the other Symbiotes go from agreeing Symbrock must die to helping them live; Even if they died, the Xenophages would just move onto the rest in their hunger. Likewise, Strickland suggests feeding all of the Symbiotes to the Xenophage to satisfy them… Cut to a scene where the Xenophage is shown to subsist just fine on human flesh as a substitute. Instead of Knull summoning more Xenophages, it’s just revealed that there were more that arrived on Earth, and they were across the globe so it took them a while to finally catch up to Eddie and Venom.
I’m not saying this is what Marcel and Hardy wrote before having to adjust for Knull’s mandated presence for the sake of fanservice. It’s just that Knull is very unobtrusive to the plot and I wouldn’t be surprised if it’s because the writers didn’t care for him (Neither do I tbh). In their sole comic appearance, the Xenophages have no connection to Knull either, debuting about twenty years before his introduction.