Adult Wolven

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

Pack Tactics. The wolven has advantage on an attack roll against a creature if at least one of the wolven's allies is within 5 feet of the creature and the ally isn't incapacitated.

Undying. Wolvens live a kind of immortal existence, whenever killed in battle the Wolven's soul turns into a shadowy outline of its form and becomes bounded to the killer, and is completely loyal to them. They made be called to fight with their master, and are summoned when their given name is spoken.

If killed in battle while bonded the Wolven can abandoned and forge a new bound. The current owner can make Wisdom saving throw to keep the Wolven DC 19. The DC decreases by 1 for every month the Wolven has been bound to its current owner.

To Summon Wolven from its shadow death state costs 10,000gp in rare monster flesh.

Shadow Form. This form can only be achieved if the Wolven has been bounded to a creature. The Wolven lives in its masters shadow, while in the shadow the wolven can see, hear and smell enhancing its masters senses. It doesn't need to sleep while in this form, but will grow weak if not fed.

Adaptability. Wolvens grow to fit their enviroment, if an Adut Wolven is in the charge of a creature it will slowly adapt that creatures alignment. The Wolven will stay loyal to its master regardless of alignment. The Wolven will begin to learn the languages spoken around it and gain the ability of speech.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 13 (4d6 + 12) piercing damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Eviscerating Howl (Recharge 5-6). The Wolven unleashes a howl in a 60ft cone that summons the the thunder of Thor. Each creature in that area must make a DC 21 Constituiton saving throw, taking 63 (18d6) thunder damage on a failed save, or half as much damage on a successful one. 

Lighting Charged. The wolven has 4 charges of lighting damage administerd through its bite. These charges do 2d12 lighting damage. These charges come back on a long rest.

Fear. Wolvens battled against the forces of the Abyss for hundreds of years. Any demon that meets the Wolvens gaze must make a charisma saving throw DC 16 saving throw or be frightened. The demon can repeat the save at the end of its turn, and is immune to the gaze if it saves.