Amphibious. The giant can breathe air and water.

Armor of Ice. When a creature within 5 feet of the giant hits it with a melee weapon attack, if the attacker doesn't have resistance or immunity to cold damage, its speed is reduced by 20 feet and it has disadvantage on weapon attack rolls until the end of its next turn.

Shapechanger. The giant can use its action to polymorph into a giant owl, killer whale, mammoth, or winter wolf, or back into its true form. Its statistics, other than its size and speed, are the same in each form. Any equipment it is wearing or carrying is transformed along with it. It reverts to its true form if it dies.

Snow Camouflage. The giant has advantage on Dexterity (Stealth) checks made to hide in snowy terrain.

Spellcasting. The giant is a 15th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 17, +9 to hit with spell attacks). It has the following druid spells prepared:

Cantrips (at-will): druidcraft, frostbite, ray of frost, shape water
1st level (4 slots): cure wounds, fog cloud, ice knife
2nd level (3 slots): gust of wind, hold person, ice sculpture (new) 
3rd level (3 slots): cold snap (new), sleet storm, slow
4th level (3 slots): control water, frost armor (new), ice storm
5th level (2 slots): cone of cold, control winds, hold monster
6th level (1 slot): wall of ice
7th level (1 slot): globe of winter (new) 
8th level (1 slot): control weather, encasing ice (new)

War Caster. The giant has advantage on Constitution saving throws that it makes to maintain its concentration on a spell when it takes damage. It also can perform the somatic components of spells even when it has weapons or a shield in one or both hands.

Wintry Cryomancy. When the giant casts a spell of 1st-level or higher, ground in the spell's area is covered with slick ice until the end of the its next turn, making it difficult terrain. Whenever a creature enters the area for the first time on a turn or starts its turn there, it must succeed on a DC 17 Dexterity saving throw, or else it falls prone. Also, whenever the giant deals cold damage to a creature, that creature's speed is reduced by 10 feet until the end of its next turn.

Multiattack. The giant makes two melee attacks.

Ice Flail (Talons in Owl Form, Bite in Whale or Wolf Form, Gore in Mammoth Form). Melee Weapon Attack: +11 to hit, reach 10 ft. (5 ft. in owl, whale, or wolf form), one target. Hit: 28 (5d8 + 6) bludgeoning damage (piercing in beast form) plus 9 (2d8) cold damage.

Frozen Rock (Giant Form Only). Ranged Weapon Attack: +11 to hit, range 60/240 ft., one target. Hit: 29 (4d10 + 6) bludgeoning damage plus 18 (4d8) cold damage.

Cold Breath (Recharge 5–6). The giant exhales a blast of freezing wind in a 60-foot cone. Each creature in that area must make a DC 18 Constitution saving throw, taking 36 (8d8) cold damage on a failed save, or half as much damage on a successful one. The ground in the area is affected by the giant's Wintry Cryomancy trait as if this was a spell.