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

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

Blood Scent. Selenelion can follow a scent through phase shifts, ethereal movement, dimension door, and fey step of any kind. Teleport and plane shift are beyond the celestials ability to follow.

Magical Attacks. Selenelions attacks are magical and overcome nonmagical resistance

Multi Attack. Selenelion can make two Attacks, one claw and a bite. If both attacks hit then the Celestial can attempt to grapple the creature by a contested strength check.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 16 (2d10 + 5) piercing damage. If the target is a creature, it must succeed on a DC 15 Strength saving throw or be knocked prone.

Claw. Melee weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 12 (2d6+5) Slashing damage.

Vorpal Bite. Selenelion is imbued with the razor sharp teeth of its father Fenris, on a critical hit the target must succeed a DC 14 Strength saving throw or lose an appendage. 

Howl of the Fenris (3/day). Selenelion can let out a howl mimicking their father, which rattles the will of any creature within 100 feet of Selenelion that can hear it. Each creature much make a DC 15 wisdom saving throw of be frightened and have disadvantage on wisdom throws and ability checks for the next minute. If a creature saves from the howl, they are immune to the effect for the next 24 hours.

As a reaction when Selenelion is hit with a melee attack that it can see, the wolf ignites with a protective flaming Purple aura around its body adding 3 to its AC against the attack.