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

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

Multiattack. The gwyllgi makes 3 hellfang attacks.

Hellfang. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) piercing damage and 3 (1d6) fire damage. If the target is a creature, it must succeed on a DC 13 Strength saving throw or be knocked prone.