Aethedorah

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

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

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

Electrified Body. The kaiju’s attacks deal an additional 3d6 lightning damage on a hit (included in the attack). In addition, each time the kaiju takes damage from a melee attack, the attacker suffers 3d6 lightning damage.

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

Multiattack. The kaiju can use its frightful presence. It then makes five attacks: one with its bite, two with its claws, one with constrict and one with its tail. It can replace its bite with swallow.

Frightful Presence. Each creature of the kaiju’s choice within 1 mile of it and aware of it 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 kaiju 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 kaiju’s Frightful Presence for the next 24 hours.

Bite. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 22 (3d12 + 6) piercing damage plus 10 (3d6) lightning damage. The target is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the kaiju can't bite another target.

Constrict. Melee Weapon Attack: +18 to hit, reach 5 ft., one creature. Hit: 22 (2d12 + 9) bludgeoning damage plus 22 (2d12 + 9) slashing damage plus 10 (3d6) lightning damage. The target is grappled (escape DC 25) if the behir isn't already constricting a creature, and the target is restrained until this grapple ends.

Tail Whip. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 24 (4d6+10) bludgeoning damage and 10 (3d6) lightning damage, If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Lightning Breath (Recharge 5–6). The kaiju exhales a line of lightning that is 20 feet long and 5 feet wide. Each creature in that line must make a DC 25 Dexterity saving throw, taking 78(12d12) lightning damage on a failed save, or half as much damage on a successful one.

Swallow. The kaiju makes one bite attack against a Huge or smaller target it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the behir, and it takes 27 (6d8) acid damage at the start of each of the kaiju’s turns. A kaiju can have only one creature swallowed at a time.

If the kaiju takes 30 damage or more on a single turn from the swallowed creature, the kaiju must succeed on a DC 14 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the behir. If the kaiju dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.

Breath Weapon (Recharge 5-6). The kaiju exhales a line of energy that is 10 feet wide and 1 mile long. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) lightning damage on a failed save, or half as much damage on a successful one.

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

Attack. The kaiju makes one attack (except bite).

Move. The kaiju moves up to half its speed.

Chomp. (Costs 2 Actions). The kaiju makes one bite attack or uses it’s swallow (if available).