Keen Smell. The undead frost troll has advantage on Wisdom (Perception) checks that rely on smell.

Undead Fortitude. If damage reduces the undead frost troll to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the undead frost troll drops to 1 hit point instead.

Undead Nature. The undead frost troll doesn't require air, food, drink, or sleep.

Multiattack. The undead frost troll makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.