#thirdtofifth fiend

Kelvezu
Medium fiend (demon), chaotic evil
Armor Class 19 (natural armor)
Hit Points 217 (29d8 + 87)
Speed30 ft., fly 60 ft.
Str21,Dex24,Con16,Int17,Wis16,Cha16
Saving Throws Dex +13, Int +9, Wis +9
SkillsStealth +13
Damage Immunities poison
Damage Resistances cold, fire, lightning; bludgeoning, piercing, and slashing from nonmagical attacks
Condition Immunitiespoisoned
Sensestruesight 120 ft. passive Perception 13
LanguagesAbyssal, telepathy 120 ft.
Challenge18 (20000 XP)
Evasion. Whenever the kelvezu is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, the kelvezu instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.
Magic Resistance. The kelvezu has advantage on saving throws against spells and other magical effects.
Magic Weapons. The kelvezu’s weapon attacks are magical.
Sneak Attack (1/Turn). The kelvezu deals an extra 28 (8d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the kelvezu that isn’t incapacitated and the kelvezu doesn’t have disadvantage on the attack roll.
Actions
Multiattack. The kelvezu makes five attacks: three with its scimitar, and two with its dagger.
Scimitar.Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 10 (1d6+7) slashing damage and 7 (2d6) poison damage. If the target is a creature other than an undead or a construct, it must succeed on a DC 17 Constitution saving throw or lose 4 (1d8) hit points at the start of each of its turns due to a bleeding wound. Each time the kelvezu hits the wounded target with this attack, the damage dealt by the wound increases by 4 (1d8). Any creature can take an action to stanch the wound with a successful DC 17 Wisdom (Medicine) check. The wound also closes if the target receives magical healing.
Dagger.Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 9 (1d4+7) piercing damage and 7 (2d6) poison damage. If the target is a creature other than an undead or a construct, it must succeed on a DC 17 Constitution saving throw or lose 4 (1d8) hit points at the start of each of its turns due to a bleeding wound. Each time the kelvezu hits the wounded target with this attack, the damage dealt by the wound increases by 4 (1d8). Any creature can take an action to stanch the wound with a successful DC 17 Wisdom (Medicine) check. The wound also closes if the target receives magical healing.
Teleport.The kelvezu magically teleports, along with any equipment it is wearing or carrying, up to 120 feet to an unoccupied space it can see.

If mariliths are the generals of Abyssal armies, and balors the kings of Abyssal realms, then kelvezus are the assassins and spies of the Abyssal realms. Rather than leading other demons into battle, they serve as agents of elimination, almost always working alone. While not able to magically disguise itself, its human-like shape makes it quite easy for it to disguise itself as a human or tiefling with just a tinge of work from a disguise kit. Kelvezus are around the same height and weight as a human. 

Originally from the Monster Manual II.

Desert Devil
Medium fiend (devil), lawful evil
Armor Class 14 (natural armor)
Hit Points 105 (14d8 + 42)
Speed30 ft.
Str15,Dex15,Con16,Int6,Wis10,Cha12
Saving Throws Dex +5, Con +6, Wis +3
Damage Immunities fire, poison
Damage Resistances cold; bludgeoning, piercing, and slashing from nonmagical attacks that aren’t silvered
Sensesdarkvision 120 ft. passive Perception 10
LanguagesInfernal, telepathy 120 ft.
Challenge6 (2300 XP)
Desert Halo. Sand constantly swirls around the devil. Whenever a creature enters within 10 feet of the devil for the first time in a turn, or starts its turn within 10 feet of the devil, that creature takes 7 (2d6) slashing damage.
Devil’s Sight. Magical darkness doesn’t impede the devil’s darkvision.
Infernal Savagery. A melee weapon deals one extra die of its damage when the devil hits with it (included in the attack).
Magic Resistance. The devil has advantage on saving throws against spells and other magical effects.
Actions
Multiattack. The desert devil makes two scimitar attacks.
Scimitar.Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 9 (2d6+2) slashing damage.
Whirlwind Attack (Recharge 5-6). The desert devil makes one scimitar attack against each creature it can reach.

Sand constantly swirls around these creatures as they patrol the desert, excoriating their flesh. Also known as aratons, desert devils always travel alone, appearing as a mere dust cloud from far away, but as they approach it becomes clear they are something far worse.  A desert devil stands around 6 feet tall and weighs 190 lbs.

Originally from Sandstorm.