Big Fish Boss

Legendary Fish (3/Day). If the big fish boss fails a saving throw, it can choose to succeed instead. Alternatively, if the big fish boss succeeds a saving throw, it can choose to fail instead. 

Magic Fish. The big fish boss has advantage on saving throws against spells and other magical effects.

Reflective Fish. Any time the big fish boss 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 big fish boss is unaffected. On a 6, the big fish boss is unaffected, and the effect is reflected back at the caster as though it originated from the big fish boss, turning the caster into the target.

Fish Monster. The big fish boss deals triple damage to objects and structures. The big fish boss deals double damage to non-fish creatures.

Hold Breath. The big fish boss can hold its breath for 1d4 centuries. 

Multiattack. The big fish boss can use its Frightful Presence, and its Children of the Fish. It then makes three attacks: two with its pearly white bite, one with its big fin, one with its big slam. It can use a Swallow or Algae Breath instead of a bite.

Pearly White Bite. Melee Weapon Attack: +15 (fish-teen) to hit, reach 25 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the big fish boss can’t bite another target.

Big Fin. Melee Weapon Attack: +15 (fish-teen) to hit, reach 40ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Big Slam. Melee Weapon Attack: +15 (fish-teen) to hit, reach 10ft., one target. The big fish boss moves up to its speed, then takes a high jump (base 13 feet up), Hit: 4 (1d6 damage per ten feet fallen) bludgeoning damage. 

Frightful Presence. Each creature within 120 feet of the big fish boss that is not a fish must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the big fish boss, or any other fish is within line of sight, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the big fish boss’ Frightful Presence for the next 24 hours.

Children of the Fish (1/Day). The big fish boss magically spits out 2d6 reef sharks, and 1d4 giant sharks. The called creatures arrive in 1d4 rounds, launching out of the big fish bosses mouth toward nearby creatures. The creatures act as allies of the big fish boss and obey its commands. The beasts remain for 1 hour, until the big fish boss dies, or until the big fish boss dismisses them as a bonus action.

Swallow. The big fish boss makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite’s damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the big fish boss, and it takes 56 (16d6) acid damage at the start of each of the big fish boss’ turns.

If the big fish boss takes 60 damage or more on a single turn from a creature inside it, the big fish boss must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the big fish boss. If the big fish boss dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Algae Breath (Recharge 5–6). The Big Fish Boss exhales scalding  breath and stinky algae in a 60-foot cone. Each creature in that area must make a DC 22 Constitution saving throw, taking 104 (15d12) acid damage on a failed save, or half as much damage on a successful one. Smelling very nice doesn't grant resistance against this damage.

Lil’ Fin. Melee Weapon Attack: +15 (fish-teen) to hit, reach 40ft., one target. Hit: 2 (1d4 - 1) bludgeoning damage.

The big fish boss can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The big fish boss regains spent legendary actions at the start of its turn.

Big Fish Energy. The big fish uses its Innate Spell-casting to cast jump on itself. It can be under the effect of up to 3 jump spells at a time.

Something Fishy (Costs 3 Actions). The big fish boss uses its Innate Spell-casting to cast tsunami if in water, or earthquake if on land.