HERE WE GO AGAIN

You like aliens, don't ya? Would you like all of them? Welllllllllllll, this is a multiverse of monsters right here!

THE HARVESTERS

"One single flood spore can destroy a species. Were it not for the Arbiter's counsel, I would've glassed your entire planet." - Halo 3

From the sky, you'll find meteor strikes and terrible UFO pilots crashing, spreading a sickness to the wildlife. Or just a blind idiot made with a netherite shell, either one. At night, there's a high chance you'll find massive reactivated mechs of the ones whom are sending their bioweapons down. In the deserts, you have the potential to find massive "arachnids," which are the type to overthrow intergalactic governments. You could also find some weaker arachnids beforehand, since the workers get driven out of hiding by The Flood. In a new type of forest, you have some demogorgons. Yeah, horrifying, I know.

"Come on you apes, you wanna live forever?!" - Starship Troopers

Under the ground, you'll find ancient remnants of a war against these heathens from a long time ago. Bunkers are full of resources, but also zombies. Alien parasites filled with acid can find in the more uninhabitable bunkers.

THE INVASION

"You still don't understand what you're dealing with, do you? The Perfect Organism. Its structural perfection is matched only by its hostility." - Alien

There are multiple waves, although the first will be, by chance, either The Flood or The Swarm. After that, the death angels and tripods. Every four days, a special UFO will crash, carrying The Thing. Finally, biome-specific aliens will start spawning everywhere, such as the xenomorphs (specifically facehuggers). Things will start spawning naturally around this time as well.

THE WORLD AND YOU

"It's forced conforming. That's what's killing the kids. That's the real monster." - Stranger Things

The villages are populated with... humans???!!! Someone should throw a bioweaponized spore at them, ya know? Oh wait, wrong mod pack for that. Anyway, these guys also have enslaved villagers. There are much wildlife, ya know? Also, you are a viking, so no tech, you can have extra dimensions.