Multiattack. Brunin makes three attacks: one with its bite, one with its claws and one to constrict.
Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 20 Wisdom saving throw or become frightened for 1 minute. 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 24 hours.
Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 2d10 + 7 piercing damage and 1d10 lightning damage.
Claws. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 2d10 + 7 slashing damage.
Constrict. Melee Weapon Attack: +10 to hit, reach 5 ft., one Large or smaller creature. Hit: 2d10 + 6 bludgeoning damage plus 2d10 + 6 slashing damage. The target is grappled (escape DC 16) if Brunin isn't already constricting a creature, and the target is restrained until this grapple ends.
Lightning Breath (Recharge 5–6). Brunin exhales a line of lightning that is 30 feet long and 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 8d10 lightning damage on a failed save, or half as much damage on a successful one.
Swallow. Brunin 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 Brunin, and it takes 21 (6d6) acid damage at the start of each of the dragon's turns. Brunin can have only one creature swallowed at a time.
If Brunin takes 30 damage or more on a single turn from the swallowed creature, he 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 him. If the behir dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.
Detect. The dragon makes a Wisdom (Perception) check.
Tail Attack. The dragon makes a tail attack. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 1d10 + 7 slashing damage.
Lair and Lair Actions
On initiative count 20 (losing initiative ties), the dragon takes a lair action to cause one of the following effects; the dragon can’t use the same effect two rounds in a row:
- Freezing fog fills a 20-foot-radius sphere centered on a point the dragon can see within 120 feet of it. The fog spreads around corners, and its area is heavily obscured. Each creature in the fog when it appears must make a DC 10 Constitution saving throw, taking 10 (3d6) cold damage on a failed save, or half as much damage on a successful one. A creature that ends its turn in the fog takes 10 (3d6) cold damage. A wind of at least 20 miles per hour disperses the fog. The fog otherwise lasts until the dragon uses this lair action again or until the dragon dies.
- Jagged ice shards fall from the ceiling, striking up to two creatures underneath that the dragon can see within 120 feet of it. The dragon makes one ranged attack roll (+7 to hit) against each target. On a hit, the target takes 2d6 piercing damage.
- Lightning arcs, forming a 5-foot-wide line between two of the lair’s solid surfaces that the dragon can see. They must be within 120 feet of the dragon and 120 feet of each other. Each creature in that line must succeed on a DC 15 Dexterity saving throw or take 10 (3d6) lightning damage.