Pain drinker. 3 times doing combat, the aberration will regain HP with the amount of damage it deals.

Horrifying Visage. Each non-undead creature within 60 feet of the Aberration that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 2 rounds and need to repeat the wisdom saving throw. If the save fails by 5 or more, the target also ages 1d4 × 10 years. A frightened target can repeat the saving throw at the end of each of its turns, ending the frightened condition on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to this Aberation Horrifying Visage for the next 24 hours. The aging effect can be reversed with a greater restoration spell, but only within 24 hours of it occurring.

(if succeeded they will have to roll again after 6 rounds, with a dc on 10 ) 

Shadow Blend. While in dim light or darkness, the shadow mastiff can use a bonus action to become invisible, along with anything it is wearing or carrying. The invisibility lasts until the shadow mastiff uses a bonus action to end it or until the shadow mastiff attacks, is in bright light, or is incapacitated.

Claws. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 2) slashing damage. The target must succeed on a DC 10 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Whip of the spine. Is a weapon created in the process of creating this aberration, the whip is a monster as well, it deals a 2d4+4 necrotic damage, if the roll on the attack roll is 18 or above the will try to grabble itself around the target, the target needs to succeed a Dex saving throw 12 if failed the target will be grabbled and will be taking a 1d4+4 damage each turn until getting out, it will die after that attack.