Blood Frenzy. The shark has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Keen Sight. The shark has has advantage on Wisdom (Perception) checks relying on vision.

Momentum. The shark adds its Dexterity modifier to its melee attack and damage rolls (already included in the shark's attacks).

Water Breathing. The shark can breathe only underwater.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage.

Denticles. The shark’s skin is covered in razor sharp, tooth-like scales. A creature that touches the megalodon or hits it with a melee attack while within 5 feet of it takes 3 (1d6) slashing damage.

Venomous Spine. The shark has a venomous spine protruding from its  dorsal fin. Whenever a creature touches the shark or hits it with a melee attack while within 5 feet, roll a d6.  On a 1 to 4, the attack occurs normally.  On a 5 to 6, the attacker takes 3 (1d6) piercing damage plus 4 (1d8) poison damage and must succeed on a DC 15 Constitution saving throw or be poisoned until it finishes a long rest.

Water Breathing. The shark can breathe only underwater.

Bite. Melee Weapon Attack: +2 to hit, reach 5 ft., one target. Hit: 3 (1d6) piercing damage.

Blood Frenzy. The shark has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Keen Sight. The shark has has advantage on Wisdom (Perception) checks relying on vision.

Momentum. The shark adds its Dexterity modifier to its melee attack and damage rolls (already included in the shark's attacks).

Water Breathing. The shark can breathe only underwater.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage.

Blood Frenzy. The shark has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Heavy Hauler. When the shark grapples a Medium or smaller creature, its speed is not reduced.

Water Breathing. The shark can breathe only underwater.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) piercing damage.  If the target is Medium or smaller, it is grappled (escape DC 12). Until this grapple ends, the target is restrained, the shark can automatically hit the target with its bite, and the shark can't make bite attacks against other targets.

Pack Tactics. The shark has advantage on an attack roll against a creature if at least one of the shark's allies is within 5 feet of the creature and the ally isn't incapacitated.

Pack Leader.  When the shark hits a creature with a melee weapon attack, each of the shark's allies within 5 feet of that creature can use their reaction to make a melee weapon attack against that creature.

Water Breathing. The shark can breathe only underwater.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) piercing damage.

Pack Tactics. The shark has advantage on an attack roll against a creature if at least one of the shark's allies is within 5 feet of the creature and the ally isn't incapacitated.

Spinning Charge. If the shark moves at least 20 feet straight toward a creature and then hits it with a bite attack on the same turn, the shark can make one spin attack against it as a bonus action.

Water Breathing. The shark can breathe only underwater.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) piercing damage.  If the target is Medium or smaller, it is grappled (escape DC 12).  Until this grapple ends, the target is restrained, and the shark can't make bite attacks against other targets.

Spin.  The shark spins while grappling a creature, attempting to violently thrash it in the water.  A creature the shark is grappling must make a DC 12 Strength or Dexterity saving throw (target's choice).  On a failed save, the target takes 6 (1d8 +2) piercing damage and remains grappled by the shark.  On a successful save, the target takes only half that damage and escapes from being grappled by the shark.

Blood Frenzy. The shark has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Mutualist Host.  The shark is accompanied by 6 (1d10) pilot fish or 1 swarm of pilot fish.  All pilot fish or swarms of pilot fish within 5 feet of the shark have advantage on melee attack rolls.  Additionally, while the shark has at least 1 hit point, attack rolls made against any pilot fish or swarm of pilot fish within 5 feet of it have disadvantage.

Water Breathing. The shark can breathe only underwater.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) piercing damage.

Protective Host.  Any time a pilot fish or swarm of pilot fish within 5 feet of the shark is attacked, the shark can use its reaction to move up to its speed towards the attacking creature and make 1 bite attack against it.

Mutualist.  The pilot fish may select a Medium or larger creature as a host.  When a pilot fish is within 5 feet of its host, it does not roll for initiative; instead, it takes its turn immediately after its host's.  The pilot fish moves with its host whenever it moves; on its own turn, the pilot fish's speed is reduced to 5 feet.  The pilot fish can move freely through its host's space or the space of any other creature with the Mutualist trait and same host.

Water Breathing. The pilot fish can breathe only underwater.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 5 (1d4 + 3) piercing damage.

Clean.  The pilot fish cleans its host's wounds.  It the host doesn't have all its hit points or has been afflicted with a disease or the blinded, deafened, paralyzed, or poisoned condition, the pilot fish can restore 3 (1d4 + 1) hit points to its host or end either one disease or one of the above conditions afflicting it.

Mutualist.  The swarm may select a Medium or larger creature as a host.  When the swarm is within 5 feet of its host, it does not roll for initiative; instead, it takes its turn immediately after its host's.  The swarm moves with its host whenever it moves, the swarm remaining centered on its host; on its own turn, the swarm's speed is reduced to 5 feet.  The swarm can move freely through its host's space or the space of any other creature with the Mutualist trait and same host.

Water Breathing. The swarm can breathe only underwater.

Bite.  Melee Weapon Attack: +5 to hit, reach 0 ft., one creature in the swarm's space. Hit: 14 (4d6) piercing damage, or 7 (2d6) piercing damage if the swarm has half of its hit points or fewer.

Clean.  The swarm cleans its host's wounds.  It the host doesn't have all its hit points or has been afflicted with a disease or the blinded, deafened, paralyzed, or poisoned condition, the swarm can restore 11 (4d4 + 1) hit points, or 6 (2d4 + 1) hit points if the swarm has half of its hit points or fewer, to its host or end either one disease or one of the above conditions afflicting it.

Blood Frenzy.  The onchopristis has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Hacksaw. The first time the onchopristis hits a creature with its rostrum during the current round, it can make a melee weapon attack against the same target as a bonus action.

Sweeping Blows.  When the onchopristis makes a melee weapon attack against a creature, that attack also targets all creatures within 5 feet of target.  Use the original attack and damage roll for all targets.

Water Breathing. The onchopristis can breathe only underwater.

Rostrum. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 14 (3d6 + 4) piercing damage.

Water Breathing. The shark can breathe only underwater.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) piercing damage. If the target is a creature, it must succeed on a DC 14 Constitution saving throw against disease or become poisoned until the disease is cured. Every 24 hours that elapse, the target must repeat the saving throw, reducing its hit point maximum by 5 (1d10) on a failure. The disease is cured on a success. The target dies if the disease reduces its hit point maximum to 0. This reduction to the target's hit point maximum lasts until the disease is cured.

Blood Frenzy. The shark has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Innate Spellcasting. The shark's innate spellcasting ability is Wisdom (spell save DC 14). The shark can innately cast the following spells, requiring no material components:

At will: color spray, dancing lights, detect magic

3/day each: confusion, hypnotic pattern (centered on self), invisibility

Prismatic Immunity. The shark is immune to any light-based spell such as color spray and prismatic spray.  Additionally at the start of each of the shark's turns, roll a d8.  The shark gains damage immunity for the current round based on the result.  1 = fire, 2 = acid, 3 = lightning, 4 = poison, 5 = cold, 6 = psychic, 7 = necrotic, 8 = the shark gains immunity to 2 damage types.  Roll twice more, rerolling any 8.

Rainbow Runner. The shark can swim and breathe in rainbows as if they were water.

Radiant Absorption. Whenever the shark is subjected to radiant damage, it takes no damage and instead regains a number of hit points equal to the radiant damage dealt.

Reflective Scales. Any time the shark is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the shark is unaffected. On a 6, the shark is unaffected, and the effect is reflected back at the caster as though it originated from the shark, turning the caster into the target.

Water Breathing. The shark can breathe only underwater.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) piercing damage plus 4 (1d8) damage of a random type.  Roll a d8 to determine the damage type: 1 = fire, 2 = acid, 3 = lightning, 4 = poison, 5 = cold, 6 = psychic, 7 = necrotic, 8 = radiant.