Battle Frenzy. When Fenrir is at or below one-quarter his maximum hit points, he can make one additional attack on his turn. When it does so, he gains a +7 bonus to his damage rolls, but also gains a -7 penalty to his attack rolls.

Energetic Attack. Fenrir deals an additional 7 (2d6) necrotic damage on each of his weapon attacks (included in the attack).

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

Legendary Resistance (3/Day). If Fenrir fails a saving throw, he can choose to succeed instead. 

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

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

Primal Strike. Fenrir ignores other creatures' resistance and immunity to nonmagical bludgeoning, piercing, and slashing damage. 

Regeneration. Fenrir regains 20 hit points at the start of his turn if he has at least 1 hit point remaining.

Siege Monster. Fenrir deals double damage to objects and structures. 

Trample. When Fenrir enters any enemy's space for the first time on a turn, the enemy must succeed on a DC 22 Strength saving throw or be knocked prone. 

Multiattack. Fenrir can use his Frightful Presence. He then makes four weapon attacks.

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 21 (4d6 + 7) piercing damage plus 7 (2d6) necrotic damage. If the target is a creature, it must succeed on a DC 22 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of Fenrir's choice that is within 120 feet of him and aware of him must succeed on a DC 15 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Fenrir's Frightful Presence for the next 24 hours.

The Fenrir can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The Fenrir regains spent legendary actions at the start of its turn.

Detect. Fenrir makes a Wisdom (Perception) check.

Move. Fenrir moves up to half his speed.

Attack. Fenrir makes one weapon attack.