Charybdis

Amphibious Charybdis can breathe air and water

Freedom of Movement Charybdis ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. It can spend 5 feet of movement to escape from nonmagical restraints or being grappled

Siege Monster Charybdis deals double damage to objects and structures.

Monumental Charybdis measures 100ft by 250ft

Multiattack. Charybdis can use its frightful presence. It then makes 4 Tentacle attacks, any of which it can replace with a fling, bite or swallow attack.

Frightful Presence Each creature of the Charybdis's choice that is within 120 feet of it and aware of it must succeed on a DC 19 Wisdom saving throw or become frightened for 5 minutes. A creature can repeat the saving throw at the end of each of its turns, 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 dragon's Frightful Presence for the next 2 hours.

Tentacle Melee Weapon Attack: +18 to hit, reach 40 ft., one target. Hit: 29 (4d6 + 15) [bludgeoning] damage. Charybdis has 18 tentacles and can grapple any creature of Gargantuan or smaller at the same time.

Fling One creature of Gargantuan or smaller is thrown up to 100ft and knocked prone. If the thrown creature strikes a solid surface, it must take 4 (1d8) [Bludgeoning] damage for every 10ft thrown. 

Swallow Charybdis 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 Charybdis, and it takes 56 (16d6) acid damage at the start of each of Charybdis' turns.

If Charybdis takes 60 damage or more on a single turn from a creature inside it, Charybdis must succeed on a DC 18 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Charybdis. If Charybdis dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Bite Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: 42 (5d12 + 10) [piercing] damage.

Suction (Recharge: 3 actions) A cone 50ft wide and 80ft long of water is drawn into Charybdis's mouth. All creatures within that area must take a DC 21 dexterity saving throw or be swallowed as if the swallow attack was used.

Whirlpool If a ship passes above Charybdis, it will automatically use the mythic action: Whirlpool.

Enter the description for how legendary actions work for your monster here.

Legendary Action Name (Costs # Actions). Enter the description for your legendary action.

Charybdis may use any mythic action. Only one mythic action option can be used at a time and only at the end of another creature’s turn. Charybdis regains spent mythic actions at the start of its turn.

Whirlpool (Costs 5 Actions). An 250ft wide cone up to the surface of the water generates at Charybdis's mouth. All creatures within that area must take a DC 24 dexterity saving throw (which is automatically failed by creatures of 13 or less strength) or be swallowed as if the swallow attack was used. Vehicles up to 80ft in diameter are automatically swallowed. Charybdis may not use its bite, swallow or suction attacks during this time but may still use tentacle or fling.

Suction (Costs 3 actions) A cone 50ft wide and 80ft long of water is drawn into Charybdis's mouth. All creatures within that area must take a DC 21 dexterity saving throw or be swallowed as if the swallow attack was used.

Tentacle Charybdis makes one tentacle attack.

Fling Charybdis uses its fling