The "No Go Zone" is a barren island far from the coast on my map. Think of the Faroe islands I want it to feel cold, windy and dangerous, something dark looms over this island. But hat monstrosities do I place there? I thought of dragons but I want something darker less "fantasy" and grimmer, any suggestions?

When it comes to dark and mysterious, my thoughts always go to aberrations! Mind flayers are my go-to for creepy-dark but there's plenty of other aberrations out there!

Oh i see, something wicked is always nice and moody.

Oozes, puddings, molds, slimes.. Nice wholesome stuff like that.

Maybe it's where adventurers go to retire. They can be rather monstrous (and dangerous) by later levels. ;)

For real though, I love subverting player expectations on stuff like this. They expect dragons/other dangerous monsters? Maybe it wasn't ever dangerous at all and was instead just a rumor spread by people who escaped there to flee from something. Or a rumor spread by a wizard/artificer/etc doing some unspeakable things in their research and wanting to keep prying eyes away.

Or it's both, and the monsters are a product of the research.

Damn, actually there's another island, "wanderers last" with that exact concept, loads of high-level characters and the HQ of the adventurers guild. 

Don't tease my appetite like that, all them oozy molds make me hungry

Quote from Amidon >>

Maybe it's where adventurers go to retire. They can be rather monstrous (and dangerous) by later levels. ;)

For real though, I love subverting player expectations on stuff like this. They expect dragons/other dangerous monsters? Maybe it wasn't ever dangerous at all and was instead just a rumor spread by people who escaped there to flee from something. Or a rumor spread by a wizard/artificer/etc doing some unspeakable things in their research and wanting to keep prying eyes away.

Or it's both, and the monsters are a product of the research.

 You've got a creative mind I'll give you that, imma use this in my other place, "wanderers last"

Aboleths, Ixitxachitl, Chuul, Skum, and Morkoth are other good options if you want to go with aquatic Aberrations for a more eldritch horror, alien theme. 

You could also have the island be a hot spot for elemental energies that spawn all manner of creatures of water, wind and ice. Things like Water and Air Elementals, Invisible Stalkers, Water and Air Elemental Myrmidon, Frost Salamanders, or Djinni and Marids. Maybe even make the reason the island is so dangerous and hard to get to be that the island under the domain of an Ancient Leviathan or a Great Elder tempest that keep the island under the effects of a never ending storm. 

For monstrosities, maybe the island is the territory of a terrifying Kraken, or the hunting grounds of a Frost Worm, or a place of great and terrible importance guarded by an Astral Dreadnought.

You don’t want a dragon but how about a dracoliche, the ritual went horribly wrong and bound it to the island, so it has made it a fortress and then resurrects all those who die in its traps. 

Or it could be the tip of a vast underwater city that sunk into ruin, perfect for an aboleth to be waiting, scheming; its minions out in the world doing its bidding. 

That's interesting too, thank you!

I hear "Monstrosity in a Cold, Windy, Dangerous, Isolated environment" and my first thought was "Mimics". Obviously, I'm drawing inspiration from The Thing, but that doesn't quite work in D&D terms. Still, it could be interesting to have the characters find like... a seemingly abandoned fortress where anyone or anything they encounter could be treasure, could be lost artifacts... or it could be a hungry ambush predator that's been waiting patiently and is desperate for a meal.

Sounds perfect for a dark and creepy take on fey. Or, if you want to stay more conventional, undead are great for lost, desolate places.

Just as one other alternative, there's no monsters to fight at all but something ( a curse, an evil artifact, a sleeping/imprisoned monster) causes any visitors to kill themselves/each other. Maybe they slowly go mad, or are infected with greed/paranoia/etc (although be careful with this as different groups have different standards for messing with player agency). Or maybe they pass out and have to face some kind of mental trial in a dream world.

I just like the idea of them showing up on an island that's completely barren aside from maybe the ominous remains of previous adventurers, and the fight they need to win is in their minds.

On an island, far from shore, with no means of sustenance or support? The quick answer is undead of various kinds. Anything else you choose to put there needs a food supply, likely imported by regular shipping which makes the creepy island far off the coast far less creepy when sailors visit regularly to drop off food.

Quote from David42 >>

On an island, far from shore, with no means of sustenance or support? The quick answer is undead of various kinds. Anything else you choose to put there needs a food supply, likely imported by regular shipping which makes the creepy island far off the coast far less creepy when sailors visit regularly to drop off food.

 Unless you have a cleric creating food and water who keeps the creatures as pets/for research.

But generally, I tend to agree about the undead being the most practical choice. Like a mummy or lich of some ancient evil person who was sent there and imprisoned. Then someone who didn't know what they were doing meddled in things they don't understand woke the mummy, it started creating an army from the remains of the other creatures on the island, blah, blah, blah. Kind of trite, but it could work.

Maybe an island full of gibbering mouthers and similar aberrations. There is a village on the island, but it's fallen into a state of disrepair. It seems like every house simply stopped being unkept, all at the same time. But the strange thing is that there are no bodies. No blood. No graves. Because the aberrations... they didn't kill the townsfolk. They are them. In a single night, the village went silent. Well, silent save the gibbering of a thousand howl mouths, and the blinking of a million lidless eyes.

It's up to you to decide what transformed the townsfolk so horrifically... and whether the players can save them.

It feels like the perfect place for a beholder actually, all it wants is to be left alone, with its experiments. :) 

Balhannon?

Why not undead?

Imagine an island where the society had an enormous graveyard on an island.  At some point, someone screwed up and a number of valiant soldiers were sent to the death to cover someone's rear end.   Maybe one of the soldiers, an honored leader for example, knows something and was sent on a suicide mission to keep his mouth shut, and a number of other righteous soldiers also died.  The spirits rose from the grave and began to wander looking for vengeance, but they couldn't get off the island except to hitch a ride by ship, or some other contrivance you want to dream up.  The clerics continued to bring the honored dead there for burial and begin to notice something isn't right, and the ruler has the island closed.  Now only clerics go there occasionally to investigate the strange things, but they can't figure it out.  But they do figure out they have to ward the docks to prevent undead from getting off the island.