Armor Class 16 (natural armor)
Hit Points 85 (10d8 + 40)
Speed 30 ft.

Saving Throws Con +7, Wis +4, Cha +5
Skills 
Damage Resistances cold; bludgeoning, piercing, and slashing from nonmagical attacks that aren't silvered
Damage Immunities fire, poison
Condition Immunities poisoned
Senses darkvision 120 ft., passive Perception 11
Languages Infernal, telepathy 120 ft.
Challenge 8 (3900 XP)

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

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

Actions

Multiattack. The devil makes two attacks with its chains.

Chain. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) slashing damage. The target is grappled (escape DC 14) if the devil isn’t already grappling a creature. Until this grapple ends, the target is restrained and takes 7 (2d6) piercing damage at the start of each of its turns.

Animate Chains (Recharges after a Short or Long Rest). Up to four chains the devil can see within 60 feet of it magically sprout razor-edged barbs and animate under the devil’s control, provided that the chains aren’t being worn or carried. Each animated chain is an object with AC 20, 20 hit points, resistance to piercing damage, and immunity to psychic and thunder damage. When the devil uses Multiattack on its turn, it can use each animated chain to make one additional chain attack. An animated chain can grapple one creature of its own but can’t make attacks while grappling. An animated chain reverts to its inanimate state if reduced to 0 hit points or if the devil is incapacitated or dies.

Reactions

Unnerving Mask. When a creature the devil can see starts its turn within 30 feet of the devil, the devil can create the illusion that it looks like one of the creature’s departed loved ones or bitter enemies. If the creature can see the devil, it must succeed on a DC 14 Wisdom saving throw or be frightened until the end of its turn.

Armor Class 15 (natural armor)
Hit Points 110 (13d8 + 52)
Speed 30 ft.

Saving Throws Str +6, Con +7, Wis +5, Cha +5
Skills Deception +5, Insight +5, Perception +8
Damage Resistances cold; bludgeoning, piercing, and slashing from nonmagical attacks that aren't silvered
Damage Immunities fire, poison
Condition Immunities poisoned
Senses darkvision 120 ft., passive Perception 18
Languages Infernal, telepathy 120 ft.
Challenge 5 (1800 XP)

Barbed Hide. At the start of each of its turns, the barbed devil deals 5 (1d10) piercing damage to any creature grappling it.

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

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

Actions

Multiattack. The devil makes three melee attacks: one with its tail and two with its claws. Alternatively, it can use Hurl Flame twice.

Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) piercing damage.

Tail. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) piercing damage.

Hurl Flame. Ranged Spell Attack: +5 to hit, range 150 ft., one target. Hit: 10 (3d6) fire damage. If the target is a flammable object that isn’t being worn or carried, it also catches fire.

Armor Class 17 (natural armor)
Hit Points 133 (14d10 + 56)
Speed 40 ft., burrow 20 ft., fly 80 ft., swim 40 ft.

Saving Throws Dex +3, Con +7, Wis +3, Cha +4
Skills Perception +6, Stealth +3
Damage Resistances 
Damage Immunities cold
Condition Immunities 
Senses blindsight 30 ft., darkvision 120 ft., passive Perception 16
Languages Common, Draconic
Challenge 6 (2300 XP)

Ice Walk. The dragon can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn’t cost it extra moment.

Actions

Multiattack. The dragon makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) piercing damage plus 4 (1d8) cold damage.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.

Cold Breath (Recharge 5–6). The dragon exhales an icy blast in a 30-foot cone. Each creature in that area must make a DC 15 Constitution saving throw, taking 45 (10d8) cold damage on a failed save, or half as much damage on a successful one.

Armor Class 17 (natural armor)
Hit Points 33 (6d8 + 6)
Speed 30 ft., fly 60 ft., swim 30 ft.

Saving Throws Dex +4, Con +3, Wis +2, Cha +3
Skills Perception +4, Stealth +4
Damage Resistances 
Damage Immunities acid
Condition Immunities 
Senses blindsight 10 ft., darkvision 60 ft., passive Perception 14
Languages Draconic
Challenge 2 (450 XP)

Amphibious. The dragon can breathe air and water.

Actions

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (1d10 + 2) piercing damage plus 2 (1d4) acid damage.

Acid Breath (Recharge 5–6). The dragon exhales acid in a 15-foot line that is 5 feet wide. Each creature in that line must make a DC 11 Dexterity saving throw, taking 22 (5d8) acid damage on a failed save, or half as much damage on a successful one.