ISLAND SHARK
Source: Homebrew.
An island shark is one of the largest monsters known. It is a type of gargantuan, but even for one of those monsters, the island shark is truly on another scale. It is so big that it can consume an entire small island in a single bite. When an island shark appears in a region, the blood of the people of that region runs cold, for all know that the island shark's voracious appetites are difficult to fulfill, and the monster is so large as to be almost impossible to drive off.
Too Big to Believe. An island shark is larger than entire cities that it threatens. Its maw gapes so wide that it can swallow entire islands and everything on them. The monster is so large that it is impossible to fight it as a single creature.
Instead, driving off the beast requires defeating several of its body parts: one eye, its mouth, its dorsal fin, and one gill. If enough damage can be inflicted to all of these parts of its body in quick succession, it can be driven off. Even while this is being done, most creatures are so small that the shark doesn't actually engage them in combat; however, merely being close to such a behemoth is dangerous, as it continually moves, creating intense currents and other dangerous effects, bumping into nearby things as it moves, etc. Its skin is rough and abrasive, so its merest touch is dangerous.
Otherwise, an island shark will eventually feed on a land mass. It is almost twenty miles long, with a gaping maw almost five miles across. It takes about a day for the shark to take a bite, but when it does, it destroys an area of the land mass about five miles in diameter and roughly spherical in shape, killing or swallowing whole every living thing on it.
An Internal Nightmare. Creatures swallowed might be able to escape, but the island shark's belly and innards almost always contains its own ecosystem full of predators, including smaller sharks, aquatic oozes such as bloodbloater swarms, blue slimes, flotsam oozes, and sometimes even communities of aquatic folk such as sahuagin, locathah, mermaids, or avanti).
Often, the interior of an island shark has some sort of terrible monster that serves as an apex predator. Even something as large as a giant shark, ooze octopus, or the like might hunt the flooded passages within an island shark.
A Lair for Mighty Recluses. There have been several occasions where powerful individuals intentionally got swallowed in order to lair within an island shark. Whether seeking isolation or adventure, creatures such as a shou lung, dragon turtle, lich, and a renegade githyanki knight of surpassing ability have hidden themselves away inside of an island shark. By doing so, they render themselves accessible only to those daring enough to face the island shark and to accept the risk involved in being swallowed.
Island Shark Treasure. An island shark doesn't collect treasure, but its gut can contain entire civilizations' worth of loot. Such treasure is inevitably taken up by the creatures that live inside the shark, however.
Creatures living inside an island shark get a +25% bonus to their chances of having treasure. In addition, they have double the normal amount of treasure.
Island Shark Eye
Gargantuan Beast (Gargantuan, Shark), always unaligned
Armor Class 15 (natural armor)
Hit Points 512 (25d20+250)
Speed 0 ft., swim 90 ft.
 
STR 30 (+10), DEX 10 (+0), CON 30 (+10),
INT 3 (-4), WIS 12 (+1), CHA 6 (-2)
 
Condition Immunities charmed, deafened, exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
Senses blindsight 100 miles., passive Perception
Languages -
Challenge 23 (50,000 xp) Prof +7
 
Part of a Greater Whole. The eye is but a part of a truly gigantic monster. They eye fills a circular space 1,500' in diameter, behind which the rest of the shark effectively forms a landscape.
If the eye is reduced to 0 hit points, it shuts, and that part of the island shark is defeated. Only if one eye, one gill, its mouth, and its fin are all defeated within 24 hours can the island shark be driven away.
Water Breathing. The shark can breathe only water.
ACTIONS
Bump. Melee Weapon Attack: +17 to hit, reach 20 ft., up to three targets in a 30' cube. Hit: 54 (8d10+10) bludgeoning damage.
Tear. The eye emits a salty, caustic tear that is a 20' radius sphere. The tear moves 90' away from the eye. Any creature it touches as it moves must make a DC 25 Dexterity save, suffering 90 (20d8) acid damage on a failure or half that on a success.
Blink. The eye blinks, causing tremendous currents. Each creature within 120' of the eye must make a DC 25 Strength save. If it fails, it suffers 46 (8d8+10) bludgeoning damage and moves 50' in a random direction. If it succeeds, it takes half damage and moves 15' in a random direction.
Island Shark Fin
Gargantuan Beast (Gargantuan, Shark), always unaligned
Armor Class 20 (natural armor)
Hit Points 1,025 (50d20+500)
Speed 0 ft., swim 90 ft.
 
STR 30 (+10), DEX 10 (+0), CON 30 (+10),
INT 3 (-4), WIS 12 (+1), CHA 6 (-2)
 
Condition Immunities charmed, deafened, exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
Senses blindsight 100 miles., passive Perception
Languages -
Challenge 29 (135,000 xp) Prof +9
 
Part of a Greater Whole. The fin is but a part of a truly gigantic monster. The fin is a triangle with sides roughly measuring 1 mile each, beneath or above which the rest of the shark effectively forms a landscape.
If the fin is reduced to 0 hit points, it stops moving, and that part of the island shark is defeated. Only if one eye, one gill, its mouth, and its fin are all defeated within 24 hours can the island shark be driven away.
Water Breathing. The shark can breathe only water.
ACTIONS
Slam. Melee Weapon Attack: +19 to hit, reach 100 ft., up to three targets in a 30' cube. Hit: 65 (10d10+10) bludgeoning damage.
Abrasive Skin. Melee Weapon Attack: +19 to hit, reach 20 ft., each creature in a 30' cube adjacent to the shark. Hit: 46 (8d8+10) slashing damage.
Stir Currents. Each creature within 60' of the fin must make a DC 27 Strength save, suffering 35 (10d6) bludgeoning damage on a failure or half that on a success. A creature that fails its save by 5 or more is also stunned until the end of the fin's next turn.
Island Shark Gill
Gargantuan Beast (Gargantuan, Shark), always unaligned
Armor Class 18 (natural armor)
Hit Points 615 (30d20+300)
Speed 0 ft., swim 90 ft.
 
STR 30 (+10), DEX 10 (+0), CON 30 (+10),
INT 3 (-4), WIS 12 (+1), CHA 6 (-2)
 
Condition Immunities charmed, deafened, exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
Senses blindsight 100 miles., passive Perception
Languages -
Challenge 25 (75,000 xp) Prof +8
 
Part of a Greater Whole. The gill is but a part of a truly gigantic monster. They gill forms a line one mile long and 100' wide, behind which the rest of the shark effectively forms a landscape.
If the gill is reduced to 0 hit points, it stops breathing, and that part of the island shark is defeated. Only if one eye, one gill, its mouth, and its fin are all defeated within 24 hours can the island shark be driven away.
Water Breathing. The shark can breathe only water.
ACTIONS
Bump. Melee Weapon Attack: +18 to hit, reach 20 ft., up to three targets in a 30' cube. Hit: 54 (8d10+10) bludgeoning damage.
Inhale (recharges when the shark uses Exhale). Each creature within 120' of the gill must make a DC 26 Strength save, suffering 55 (10d10) bludgeoning damage on a failure or half that on a success. A creature that fails is also pulled 60' toward the gill, while a creature that succeeds is pulled 15' toward the gill. A creature that ends this turn within 5' of the gill suffers an extra 28 (8d6) slashing damage.
Exhale (recharges when the shark uses Inhale). Each creature within 120' of the gill must make a DC 26 Strength save, suffering 55 (10d10) bludgeoning damage on a failure or half that on a success. A creature that fails is also pushed 60' away from the gill, while a creature that succeeds is pushed 15' away from the gill.
Cloud of Mucus. A 20' radius sphere of mucus emerges from the gill and drifts 30' away from the gill. Any creature that is touched by the cloud must make a DC 26 Dexterity save, becoming restrained, caught by and moving with the cloud on a failure. While restrained in this way, the creature can't breathe. The cloud lasts 1 minute. At the start of each of the gill's turns, it drifts 30' further from the gill.
A restrained creature can escape by using an action to make a DC 26 Strength (Athletics) or Dexterity (Acrobatics) check.
Island Shark Mouth
Gargantuan Beast (Gargantuan, Shark), always unaligned
Armor Class 18 (natural armor)
Hit Points 1,025 (50d20+500)
Speed 0 ft., swim 90 ft.
 
STR 30 (+10), DEX 10 (+0), CON 30 (+10),
INT 3 (-4), WIS 12 (+1), CHA 6 (-2)
 
Condition Immunities charmed, deafened, exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
Senses blindsight 100 miles., passive Perception
Languages -
Challenge  31 (175,000 xp) Prof +9
 
Part of a Greater Whole. The mouth is but a part of a truly gigantic monster. The gaping maw makes up an oval roughly 2 miles across and 1 mile high, a 200' wide rim around which can be effectively attacked., behind which the rest of the shark effectively forms a landscape.
If the mouth is reduced to 0 hit points, it shuts, and that part of the island shark is defeated. Only if one eye, one gill, its mouth, and its fin are all defeated within 24 hours can the island shark be driven away.
Water Breathing. The shark can breathe only water.
ACTIONS
Tooth Scrape. Melee Weapon Attack: +19 to hit, reach 50', up to three targets in a 30' cube. Hit: 88 (12d12+10) slashing damage. If this attack scores a critical hit, the severity increases by 2d12.
Poison Breath. A 100' radius cloud of foul, poisonous gas forms in a space adjacent to the mouth, including underwater. Creatures in the cloud must make DC 27 Constitution saves, suffering 72 (16d8) poison damage on a failure or half that on a success.
Saliva Spray. A spray of caustic saliva splatters in a 90' cone emerging from the mouth. Each creature in the spray must make a DC 27 Dexterity save, suffering 60 (11d10) acid damage on a failure or half that on a success. A creature that fails its save by 5 or more puts 1 wear point each on 3 objects that it wears or carries.
REACTIONS
Rumble (1/day). When the mouth becomes blooded, it emits a tremendous noise. Each creature within 300' must make a DC 27 Con save, taking 130 (20d12) thunder damage on a failure or half that on a success.