Blue Hellhound

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

Infernal Blood. Whenever the Hell Hound is subjected to fire damage, it takes no damage and instead regains a number of hit points equal to the fire damage dealt

Keen Hearing and Smell. The hound has advantage on Wisdom (Perception) checks that rely on hearing or smell.

Multiattack. The Hell Hound can use it's frightful presence. It then makes three attacks: one with it's bite and two with its claws.

Bite. Melee weapon attack: +14 to hit, reach 10 ft., one target. Hit: 19 (2d10+8) piercing damage plus 7 (2d6) fire damage.

Claw. Melee weapon attack: +14 to hit, reach 5 ft., one target. Hit: 15 (2d6+8) slashing damage.

Tail. Melee weapon attack: +14 to hit, reach 15 ft., one target. Hit: 17 (2d8+8) bludgeoning damage.

Fire Breath (Recharge 5-6). The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6)  fire damage on a failed save, or half as much on a successful one.

The Hell Hound 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 Hell Hound regains spent legendary actions at the start of it's turn.

Detect. The Hell hound makes a Wisdom (Perception) check.
Claw Attack. The Hell Hound Makes a claw attack.
Tail Attack. The Hell Hound Makes a tail attack.