There's a reason people don't go to Tornado Island... The food is awful.
Also the constant tornadoes, the mutating petrifying shrapnel-rock creatures, and the Invisible Man-Eating Cyclops.
"How do people know that the giant invisible man only has one eye?" You might say. How, indeed. How do they know it's a giant man? Maybe it's an giant invisible ape? Perhaps it's merely some fabricated psychological manifestation of Man's innermost fears? Or just a crazy rumor made to scare people away from the island's hidden riches?
Well, a soldier and his crew went there once, on accident. The crew's dead, but the wily soldier survived. He said that they plucked out the cyclops' eye and blinded him. Apparently he wasn't invisible then. The soldier died of old age centuries ago.
Tornado Island is a graveyard of ships. They're flung from afar by twisters that suspiciously hone in on maximum casualty targets. Trade and exploration ships in particular. For whatever reason pirate ships get left alone (until they overconfidently get too close). The island is littered with their broken husks and cargo. It's a scavengers goldmine. This suits the inhabitants of the island just fine, as they'll happily prey on far-flung crushed bodies.
Aside from its horrible rocky denizens, there's a pasture ecology that's suitable for sheep. Only The Invisible Cyclops has the power to protect a herd. He guards them jealously. Some still-living castaways might manage to survive off of the stolen sheep here and there. Other than that it's grass and rocks, rocks and grass. For miles.
There are some locations of note:
The Golden Pit:
The source of mutations on the isle. A few centuries ago a Slaad made its hibernation place on the island, thinking it dangerous enough to ward away intruders. It stored a bunch of mutagenic chemical canisters around, several of which have broken and seeped into the soil by tornado-made earthquakes. The Slaad made a dungeon to protect its hibernation cave from all except other Slaad.
Slaad, however, are terrible at architecture. The underground ceiling of the final treasure chamber collapsed about a hundred years ago, revealing its bounty to the world. There's now a big pit full of riches in the middle of the island, untouched for a fifty years because obviously a pit full of treasure is a trap. The intended entrance is deeply buried. The intended final treasure room is the de facto entrance.
It's basically a Five-Room dungeon in reverse. Treasure at the beginning. Challenges at the end. There is nothing compelling a party to go deeper into the dungeon. The moment they rappel in it's all right there, for the taking.
If they decide to traverse to the end (the beginning), past all of the traps and tests, they will encounter a Magic Mouth that warns them of the dungeon ahead. If they open the final (intended first) door, it'll just collapse open depositing a bunch of sand and rocks on the poor unfortunate souls.
The Cyclops Lair
The Invisible Blind Man-Eating Cyclops makes his lair on the island, where he keeps his sheep when he's not letting them graze. The enormous boulder he used to block the cave entrance with has been split open by a ship and its ram that flew into it from a mile off. That's how all but he enter and exit now: through a broken ship that split the rock open. He just steps over it. It smells like five barns. There's plenty of straw around to feed sheep/hide in.
There's a giant wool sleeping blanket on the floor. The back is filled with treasures pilfered from tossed ships - bolts of silks, bricks of gold and silver, whale bones. The Cyclops keeps his giant-ass scythe against the walls of the cave, perhaps a little too precariously placed... There are bell alarm systems set up to inform him of escaped sheep, though they might also catch the incautious intruder.
Every day he counts up his sheep, one by one, by hand. Picking 'em up and placing them behind him. If he finds any missing he'll go out searching for them, moving the giant boulder out and back each time. If he finds too many missing he'll go berserk and start rampaging around the island, listening for things to murder-scythe.
The Fountain of Useless Epiphany
Hidden away in the rocky crags of the eastern part of the island is a magical spring gushing forth Freshwater of Useless Epiphany. Drinking from it causes someone to remember only unuseful things for 1d4 days. They will constantly spout realizations that are incredibly obvious to everyone and are absolutely no help. It will muddle their cognitive process substantially.
"Oh yes! The Sun resides in the sky!"
"When I jump up, I come down again like so!"
"If we get killed, we will surely die!"
And so on. The person is not entirely useless, but they won't come up with any cunning plans anytime soon.
Mechanically, as so: Whenever a PC under the Fountain's effects wishes to perform an action, they must declare an incredibly obvious and utterly useless epiphany related to it, which they spout as they do so. Give the player a few seconds. If they take too long they skip their action. If their epiphanies get too same-y then start to reject them.
Unfortunately, this is the only source of freshwater on the island. The only other potable liquid is sheep's milk, and that's quite dangerous to obtain.
Gravebeach of Ships
Spine-broken ships can be found anywhere on the island, from all sea-faring nations on the planet. Most of them land on its rocky southern beach. This is where the highest concentration of rock monsters lurk, waiting to petrifying and eat tornado-tossed and body-broken sailors.
It's a scavengers dream, though it is dangerous. There are literal tonnes of trade goods, unspoiled food, and weapons. Anything a ship has can be found here. More than a few warships have been broken on the rocky shore - there are probably a few ballistae (or cannons, if the setting suits) lying around that are big enough to fell a giant. The sand is full of the petrified broken limbs and bodies of unfortunate sailors. The rock monsters only eat the good parts, like petrified brains or livers. Everything else they leave behind.
Survivors of various shipwrecks have formed a motley crew of survival. At least three-quarters of them are in a Useless Epiphany stupor. The remaining organize raids on the Cyclops Lair and the Gravebeach of Ships, utilizing their dumb companions. Somebody inevitably dies every single raid. Usually it's someone who drank the Water. Replacements come in via new shipwrecks.
If there's a leader of the Castaway Camp, it's Bosun Nobs. A wily, weathered-looking man whose fat has long melted away on the island's rugged conditions. He's lean, he's mean, he's a sheep-stealing machine. Cold as ice and all about the Survival of the Fittest. Unless you steal some sheep or scavenge some supplies you're useless to him and he won't care about you in the slightest. Not unless you're a thief. "ARR YA A TEEF?!"
Making fun of the fact that he can't make the "th" sound, and that his teeth whistle when he tries, and that he's missing most of his teeth, and that this makes his 'thief' sound like 'teeth', is the surest way to get on his bad side.
Mutation List (Roll 1d8) One per Rockatrice and Castaway group. Every other Boulderlisk has one. 1-2. Invisible 2-3. Humongous: Double size, HD, HP. 4. Exploding: Upon death, explodes. Save vs. Breath for Half of 2d10 dam, 20ft rad. 5. Toxic: Attacks are Poisonous 6. Chaotic: All rolls associated with it are 1s and 20s. Flip a coin. 7. More Limbs/Heads: Gets an extra attack. 8. Extra Legs: +10' movement.
A tornado will come ripping through the area, picking up sharp rocks and debris and shredding anything nearby that's not in total cover. Rules for tornadoes may vary depending on system.
1-10: Tornado is Far: No worries. These are common.
11-16: Tornado is Too Close for Comfort: 2d10 minutes to find cover. Total 180-degree cover prevents damage. 25% chance of flying debris. Save. vs. Breath or take 2d10 damage from a ship's mast, a boulder, or some other big object.
17-20:Tornado is Right Up On Us: 2d10 minutes to find cover. Complete bunker cover prevents damage. Otherwise Save vs. Breath or get sucked up into the funnel. On success take 1d10 damage. On failure take 1d10 damage each round for 1d10 rounds. Then fall 10x(1d8-2) feet.
AC Leather (Good Dex) HD 1 HP 4
1d4 Peck + Petrification
Head Explode: 10ft rad, Attack at +2, Save vs. Petrification or random body part turns to stone.
1-2. Left Leg (Half movement)
3-4. Right Leg (Half movement)
5-6. Primary Arm (Cannot use main hand)
7-8. Offhand Arm (Cannot use off hand)
9. Torso (Dead in 1d10 days)
10. Head (Dead in 1d10 minutes)
They look like wild turkeys, except they got spiky obsidian-shard rocks for heads. They hunt in packs that run towards you and explode. Then the flimsy neckless bodies scatter away, or feast.
They explode into little rock shards that pierce armor and petrify body parts when they hit. It's a nasty and painful way to go.
AC Plate+Shield HD 4 HP 24 Sp 15'
1d8/1d8 Boulder Throw/Boulder Throw
Petrifying Gaze: 40ft cone, Save vs. Petrification or become petrified.
5 in 6 chance of ambush in rocky/beach terrain
They hide easily in the rocky terrain. Looks like a big boulder with a bunch of smaller rocks orbiting around it like a halo. The glowing green 'eyes' of if have a petrifying gaze attack. These things aren't to be messed with.
Fasrizle, The Last Slaad
Stats and Abilities as a Grey/High Level Slaad, however that functions in your campaign.
It remains dormant in its Hibernation Pod until awakened by True Love's Kiss. It says so on the side of the pod.
This is a trick. Anyone kissing The Slaad on its slimy frog lips will get a Red Slaad egg injection, via Looney-Tunes style frenching. Save vs. Paralyze if you're feeling generous. Anyone's kiss can awaken it, though. You could even have a sheep lick his mouth. It'd wake him as much as a Prince or Princess's.
When awoken, and not finding a slaad, Fasrizle will subtly try and inject someone with Slaad eggs and then escape from the island by any means necessary. Fasrizle will happily cooperate. It means more potential hosts for later. Fasrizle is tasked with repopulating this planet with Slaadi.
(A sheep or person with incubating Slaad Eggs would be worth a small fortune if brought back to civilization in some sort of stasis. Wizards would kill over it.)
* 10% chance the Scythe will get stuck and require an action to loose.
Grab: If he's got you, he gets +6 to hold on. Next Hand attacks always hit.
Permanent Invisibility: Gets your system's benefits of Invisibility
Blind: Gets your system's penalties of being Blind.
Good Hearing: Gets advantage on checks related to hearing.
He's a one-eyed, no-horned, not-flying invisible people-eater!
The ground will shake before his arrival. One will see the sand fly where he steps. He's got fine sense of hearing and a giant-ass scythe (which is not invisible) with which to harvest little man-high foes on the island. He's got the strength to pop off mens' heads with his invisible thumb like children with dandelions. He's also blind.
He forgot his name a long time ago. Drinking too much of the Useless Epiphany Water eventually overrode the part of his brain that remembered it. He's adopted the only name he can remember: Noman.
AC Unarmored HD 2 HP 10
1d6 Sailor's Weapon
Stupor: 75% chance that they are in a useless stupor 50% of the time.
They smell like crap, and most of them are in a stupor. Bosun Hobs has a 1d8 Captain's Cutlass and twice as much HD and HP.