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

Pack Tactics. The wolf has advantage on an attack roll 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.

Expertise. Ymir's proficiency bonus is doubled for any ability check it makes that uses either of the chosen proficiencies.

Jack of All Trades. Ymir can add half its proficiency bonus to any ability check it makes that uses a skill but doesn’t already include its proficiency bonus

Extra Attack. Ymir may bite twice with it's action. 

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 4) piercing damage. If the target is a creature, it must succeed on a DC 16 Strength saving throw or be knocked prone.

Helpful/Cunning Action. Ymir can use the Help, Dash, Disengage, or Hide actions as Bonus Actions