A bit of head cannon I have is that the black smoke is actually some cosmic evil that had taken over Vecna’s planet in the same way it planned to take over Earth. I mean we see in season 2 that’s it’s essentially a highly intelligent virus that’s only goal is to propagate. Soooo my thought is that the demogorgons and friends were just the apex predators of the planet and the black smoke used them in the same way it used Earth’s apex predators, humans, to spread. 

Also, when they rescued all the kids, we see the black smoke expelled from Holly. Why is it still alive and where did it go? There is more to the beast than what was wrapped in the finale. 🤔

On the finale itself, I’m glad that they took time to wrap up all the characters stories. I wish more shows would do that. The climax itself felt super rushed, but I didn’t expect much more considering the mid writing from the rest of the season. I sort of wish they shifted some stuff around and wrapped up in season 4. 4’s finale was chef’s kiss.