D&D 5E Help me populate my world with Monsters

[Quajek]

My PCs are working as Scouts for a famous Monster Hunter.

He can't be everywhere at once, so when someone hires him to slay a terrible monster, his team of Scouts heads out overland to find the monster, confirm its existence, and locate its lair. Then the REAL Monster Hunter comes in to do his work, get the glory, and the gold.

So, I need BIG SCARY MONSTERS throughout the world, and I want to place them in areas that make sense and have other monsters around them in ways that make sense.

Basically, what I’m looking to have in terms of gameplay experience is the Monster Hunter will get a message from some far-off place. He’ll tell the PCs:

“'VILLAGE A' is a small town a stone’s throw from the edge of 'BIOME B'. They've been having 'PROBLEM C'. Some townsfolk think that 'MONSTER D' is responsible,"​

So they go check it out, but about half the time it turns out it was 'MONSTER E' or 'COMBINATION OF MONSTERS F' -- and the F is for FUN!

So, since the players are not going to be the ones actually FIGHTING the BIG monster, (at least, not at first, or not for long) what I’m looking to do is populate the region around the big monster with other dangers for the players to overcome.

Think of a target. The Bull’s Eye is the Monster they’re looking for, but in every other band on the target, they have to deal with other monsters / hazards in their way.

So, for example, they head into the Swamp to find the Black Dragon the nearby villagers are sure lives there and has been killing farmers and eating their livestock.

So, the Swamp is set up something along the lines of:

Problem: Slain farmers, stolen livestock

Swamp Hazards: Giant Insects > Snakes > Quicksand > Alligators > Jungle Goblins > Dire Alligators > Vine Blights > Lizardmen that worship the > Acid-Spitting Humongous Black-Scaled Snake

PCs Find: The Lizardmen were stealing the livestock to feed the Snake, and using weapons laced with her acid-venom to kill the farmers.​

I know I want to put a bunch of undead in a Desert, probably centered around a Mummy Lord.

But other than that, I've got every other sort of environment possible, and I need to fill them all up with monsters.

I don't just want to have a bunch of different dragons spread all over the map. The nature of what I'm looking for is variety, to give the PCs the opportunity to use every skill and spell at their disposal, to really push their wits to the edge. I want to create a UNIQUE experience for my friends.

So, please: help me place an APEX PREDATOR in every biome and then surround it with the less deadly, but no less monstrous monsters, people, terrain hazards, and creatures that would pose a threat to the lives of my players.

All help, suggestions, ideas, and advice will be accepted with open arms.

 

[MonkeezOnFire]

If you have the DMG one of the appendices contains monsters listed by biome and within each biome they are sorted by CR. It's a really great resource for something like this.

Arctic:
Apex: Remorhaz, Abominable Yeti, Frost Giants
Others: Mammoths, saber tooth tigers, polar bears, Earth elemental re-flavored to be made out of ice, young yeti

Mountain/Hills:
Apex: Chimera, Cyclops, Roc, Stone Giants, Wyvern, Bullette, Manticore (at CR 3 could even be a smaller encounter)
Others: Orcs, mountain lions, Ettin, bears, earth elementals, perytons, basilisk

Coast/Ocean:
Apex: Kraken, Dragon Turtle, Roc, Marid/Djinni
Others: Sea Hag, Harpy, sahuagin, water elementals, sharks

 

[Beleriphon]

Mountain valley, light forest, winter

Problem: Less and less game is present in the valley and the locals are seeing packs of ghost wolves
Hazards: Wild animals > cold weather > blizzards > goblins
PCs Find: the ghost wolves are actually a small pack of blink dogs hunting a particularly voracious pride of displacer beasts.

 

[neobolts]

Any of the big monsters that "employ" lesser monsters seem like a good choice. Devils, dragons, and mind flayers are good examples of this. The current MM does a good job of suggesting underlings.

 

[Wik]

I'm a fan of the twist.

So, forest setting. PCs hear that during a full moon, a bunch of wolves are mauling peasants. They get to the village, do a bit of investigating, and encounter rabid wolves. Naturally, they assume werewolves. Turns out, no, it's actually a vampire that can control wolves, or an angry druid, or a little girl in the village with a cursed magic item that's summoning the wolves.

I also like the idea of a bait and switch. Where the monster is actually deliberately setting the monster hunters up. There's a great old Margaret Weis story in DRAGON that involves a bunch of high level heroes going on a dragon hunt. They sneak into the lair, overcome the guardians, only to find out that the narrator of the story is actually the (polymorphed) Dragon, who quickly kills them. Turns out the Dragon wanted to test his security, and knew that the heroes were going to go after him anyways, so it'd be best to get the drop on them. After removing them, the dragon knew he could afford to rest easy for a decade or two.

 

[Quickleaf]

Taiga (arboreal coniferous snowforest)

Problem: Men have been wandering off into the taiga as if lured by some unearthly song, and hunters believe an Elk of Ill Omen is responsible.
Hazards: Wolves > cold weather > frigid lakes > rusulkas (creepy singing female vampire spawn disguised as water nymphs)
PCs Find: The elk (giant elk, modified shadow dragon template) is actually the animal companion of a frost giant ranger who is unhappily married to the rusulkas by virtue of an oath he made to his dying brother.