BBEGg

Legendary Resistance (3/Day). If the BBEGg fails a saving throw, it can choose to succeed instead.

Magic Resistance. The BBEGg has advantage on saving throws against spells and other magical effects.

Fury of the Uncookable. Any creature that is within 20 ft of the BBEGg during its turn takes 1d10 [Fire] damage as the BBEGg tries to cook itself

Multi Attack. The BBEGg is able to make 5 attacks as part of its attack action. It must make a minimum of 1 Shell Smash. It also may make the Yolk Swamp attack as a part of the attack action

Shell Smash. Melee Weapon Attack: +19 to hit, reach 5 ft., 1 target. Hit: 32 (4d10 + 10) [Bludgeoning] damage. 

Albumen Grab. RangedWeapon Attack: +19 to hit, range 20 ft., 1 target. Hit: 24 (4d6 + 10) [Bludgeoning] damage.

Yolk Swamp [Recharges on 4-6].  SavingThrow range 60 ft., 15 ft Radius, Hit: 32 (4d8) [Fire] +  (4d8) [Cold] damage.

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

The BBEGg 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 tarrasque regains spent legendary actions at the start of its turn.

Attack . The BBEGg makes one Shell Smash attack.

Move. The BBEGg moves up to half its speed, this also actives the Fury of the Uncookable ability

Yolk Swamp (Costs 2 actions). The BBEGg forcefully recharges its Yolk Swamp ability

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

Mythic Action Name (Costs # Actions). Enter the description for your mythic action.