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

Charge. If the Sekolah moves at least 20 feet straight toward a target and then hits it with a ram attack on the same turn, the target takes an extra 44 (8d10) bludgeoning damage. If the target is a creature, it must succeed on a DC 23 Strength saving throw or be pushed up to 20 feet away and knocked prone. 

Devil's Sight. Magical darkness doesn't impede the devil's darkvision.

Innate Spellcasting. The Sekolah's innate spellcasting ability is Wisdom (spell save DC 20, +12 to hit with spell attacks). It can innately cast the following spells, requiring no material components: 

At will: alter self (can become Medium when changing his appearance), detect magic, geas, ice storm, invisibility (self only), suggestion, wall of ice.
3/day each: locate creature, , cone of cold.

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

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

Magic Weapons. The Sekolah's weapon attacks are magical

Shark Telepathy. The Sekolah can magically command any shark within 120 feet of it, using a limited telepathy.

Sahuagin Telepathy. The Sekolah can magically command any sahuagin within 120 feet of it, using a limited telepathy.

Siege Monster. The Sekolah deals double damage to objects and structures.

Multiattack. The sekolah makes three melee attacks: one with its bite and two with its tail. It can use Ram in place of any melee attack.

Bite. Melee Weapon Attack:+ 13 to hit, reach 10ft., one target. Hit: 29 (4d10 + 7) piercing. If the target is a creature, it is grappled (escape DC 23). Until this grapple ends, the target is restrained, and the Sekolah can't bite another target. 

Ram. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 51 (8d10 + 7) bludgeoning damage.  

Tail. Melee Weapon Attack:+ 13 to hit, reach 20ft., one target. Hit: 24 (6d6 + 7) bludgeoning damage. 

Swallow. The Sekolah makes one bite attack against a large or smaller creature it is grappling. If the attack hits, that creature takes the bite's damage and 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 Sekolah, and it takes 42 (12d6) acid damage at the start of each of the Sekolah's turns.
If the Sekolah takes 50 damage or more on a single turn from a creature inside it, the Sekolah must succeed on a DC 23 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet oft he Sekolah. If the Sekolah dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Tidal Wave (Recharge 6). While submerged, the Sekolah magically creates a wall of water centered on itself. The wall is up  250 feet long, up to 250 feet high, and up to 50 feet thick.  When the wall appears, all other creatures within its area must each make a DC 23 Strength saving throw. A creature takes 33 (6d10) bludgeoning damage on failed save, or half as much damage on a successful one.
At the start of each of the Sekolah's turns after the wall appears, the wall,along with any other creatures in it ,moves 50 feet away from the Sekolah. Any Huge or smaller creature inside the wall or whose space the wall enters when it moves must succeed on a DC 23 Strength saving throw or take 27 (5d10) bludgeoning damage. A creature takes this damage no more than once on a turn. At the end of each turn the wall moves, the wall's height is reduced by 50 feet, and the damage creatures take from the wall on subsequent rounds is reduced by 1d10. When the wall reaches 0 feet in height, the effect ends.
A creature caught in the wall can move by swimming. Because of the force of the wave, though, the creature must make a successful DC 23 Strength (Athletics) check to swim at all during that turn.

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

Attack. The Sekolah makes one ram attack or tail attack.

Detect. The Sekolah makes a Wisdom (Perception) check.

Move. The Sekolah moves up to half its speed.

Chomp (Costs 2 Actions). The Sekolah makes one bite attack or uses its Swallow.