Keen Smell. The werebear has advantage on Wisdom (Perception) checks that rely on smell.

Mountain Born. The werebear is acclimated to high altitude, including elevations above 20,000 feet.

Oyaminartok's Eyepatch. Oyaminartok's eyepatch is a common wondrous item that grants her a +3 bonus to all Perception skill checks, a +3 bonus to her Passive Perception, and the ability to determine if any creature she can see is a shapechanger.

Shapechanger. The werebear can use its action to polymorph into a Medium bear-humanoid hybrid, Large bear–humanoid hybrid, or into a Large polar bear, or back into its goliath form. Its statistics, other than its size and AC, are the same in each form. Any equipment it is wearing or carrying isn’t transformed. It reverts to its true form if it dies.

Action Surge (Recharges when Oyaminartok Finishes a Short or Long Rest). On her turn, Oyaminartok can take one additional action.

Reckless. At the start of her  turn, Oyaminartok can gain advantage on all melee weapon attack rolls during that turn, but attack rolls against her have advantage until the start of her next turn.

Multiattack. The werebear makes two melee attacks

Bite (Bear or Hybrid Form Only). Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 16 (2d10 + 5) piercing damage. If the target is a humanoid, it must succeed on a DC 15 Constitution saving throw or be cursed with werebear lycanthropy.

Claw (Bear or Hybrid Form Only). Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 5) slashing damage.

Greatspear (Humanoid or Hybrid Form Only). Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 11 (1d12 + 5) piercing damage.

Rage (Recharges after a Short or Long Rest). As a bonus action, Oyaminartok can enter a rage that lasts for 1 minute. While raging, Oyaminartok gains the following benefits:

• She has advantage on Strength checks and Strength saving throws.
• She gains a +2 damage bonus to all melee weapon attack using Strength.
• She has resistance to magical bludgeoning, piercing, and slashing damage that would bypass her Werebear immunity.

She does not have resistance to bludgeoning, piercing, and slashing damage from silvered weapons. The rage ends early if Oyaminartok is knocked unconscious or if her turn ends and she hasn't attacked a hostile creature or taken damage since her last turn. 

Stone’s Endurance (Recharges after a Short or Long Rest). When the werebear takes damage, it reduces the damage taken by 10 (1d12 + 4).