I saw a few of this floating around recently, so I thought I should try one out. This is a world that I am running a campaign in IRL, but I don't have much world info yet.

I will organize suggestions in the categories below:

World Name: Kyzterram

BBEG Monster Name

Time/Tech: Typical fantasy time (medieval), medieval tech, normal magic (but divine magic isn't as common)

Religion

Geography

City/towns

Locations

History

Organizations

Homebrew races, classes, etc

Word "seed:'

[insert monster name] awoken in the depths of Kyzterram. This monster warped reality. Humanity used to rule the world, but [monster name] granted the ability for animals and plants to suddenly mutate and change into monstrosities. Any organism changed in the is way is hyper-aggressive towards humanoids. Now, humanity cowers behind their walls as monsters roam unchallenged.

Thanks for any help!

What is the level of technology and magic 

Well, if the monster is going to be the BBEG, make sure it has a goal, a plan to get there, and make sure it has a reason to be there. Ex: you could do a Lorax thing, where humanoids have been the dominant species, but due to their increased activity and their subjugation of the wildlife, they accidentally summoned an age old 'monster' that is some form of vengeful forest spirit. The world would become post apocalyptic, probably with overgrown towns and destroyed civilizations.

Some cool monster/animal ideas:

(pro tip, just smoosh different animals together. Think AtlA)

Caiman shark: Crocodile-like creatures that burrow beneath the surface. They have shark-like fins on their backs that split though the ground. Carnivorous and highly aggressive. These animals tend towards ambush hunting tactics.

Scorpion Wolf: Packs of large wolves that have evolved to grow stinger tails prowl the forests. They trap and incapacitate their prey with their paralyzing sting.

Eagle Turtle: Giant eagles with thick, armored shells. They can't ascend to the higher altitudes that other birds do, but they make up for speed with brute strength and defensive power. These birds target larger, slower moving prey. They're known for their keen eyesight.

Change Log: tweaked first post.

I had a lot more info on my first post, but it was accidentally erased before I posted it. It made me very annoyed, I I didn't feel like typing it all again at that moment. I'll go back and edit in the stuff I wrote later.

The "Lorax" thing mentioned above is what I as going for. I'll edit it in when I have a bit more time.

To expound on my first post, the players are stranded on an island, and they have amnesia. They don't know about the world, so I have some time to finish fleshing out the world.  I promise I'm not a lazy DM 😂

Quote from Dimir_MTG >>

Now, humanity cowers behind their walls as monsters roam unchallenged.

 I would go with *soon* humanity will cower. You want to build tension. I don't know where you can go from here. It would be better to start with the effect taking place in limited parts of the world, and slowly spreading. That way you can start with small challenges and hints of what's happening, and unveil and intensify it through the campaign.