CHA 12 (+1) CON 20 (+5) DEX 14 (+2) INT 11 (+0) STR 25 (+7) WIS 16 (+3)
Damage Immunities: cold; piercing, bludgeoning, slashing from nonmagical weapons
Damage Resistances: fire, lightning, acid
Condition Immunities: frightened
Languages: understands Draconic and Common but cannot speak, telepathy 120ft
Senses: Darkvision 90ft, Tremorsense 60ft
Skills: Perception +9, Stealth +8
Amphibious: The behir can breathe both air and water.
Underwater Camouflage: The behir has advantage on Dexterity (Stealth) checks made while underwater.
Legendary Resistances (3/day): If the behir fails a saving throw, it can choose to succeed instead.
Multiattack: The behir makes two attacks: one with its bite and one to constrict.
Bite: Melee Weapon Attack: +10 to hit, reach 10ft, one target. Hit: 23 (3d10 + 7) piercing damage.
Constrict: Melee Weapon Attack: +10 to hit, reach 5 ft, one Large or smaller creature. Hit: 20 (3d8 + 7) bludgeoning damage plus 20 (3d8 + 7) slashing damage. The target is Grappled (escape DC 19) if the behir isn't already constricting a creature, and the target is Restrained until this grapple ends.
Whirpool Breath (Recharge 5-6): The behir inhales a whirpool that is a 60ft cone. Each creature in that cone must make a DC 19 Strength saving throw, taking 54 (12d8) bludgeoning damage on a failed save and being pulled 15ft toward the behir. On a successful save the creature takes half as much damage and is not pulled.
Swallow: The behir makes one bite Attack against a Medium 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 21 (6d6) acid damage at the start of each of the behir's turns. A behir can have only one creature swallowed at a time.
If the behir takes 50 damage or more on a single turn from the swallowed creature, the behir must succeed on a DC 15 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10ft of the behir. If the behir dies, a swallowed creature is no longer Restrained by it and can escape from the corpse by using 15ft of Movement, exiting prone.
The oceanic behir can take 3 legendary actions choosing from the options below. Only one legendary actions option can be used at a time and only at the end of another creature’s turn. The behir regains spent legendary actions at the start of its turn.
Quick Dash: Behir moves up to half its movement speed without provoking opportunity attacks.
Bite (Costs 2 Actions): The behir can take a single melee bite attack. Melee Weapon Attack: +10 to hit, reach 10ft, one target. Hit: 23 (3d10 + 7) piercing damage.
Tail Whip (Costs 3 Actions): The behir can make a Melee Weapon Attack: +10 to hit, reach 15 ft, one target. Hit: 29 (4d10 + 7) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be pushed up to 10ft away from the behir and knocked prone.
Homebrew monster design is all @enemytosleep; illustrated by me. Custom statblock made for Chapter 2 of our Critical Role fic, Into the Deep. Please go ahead and torment your players with this long boi.