Ancient White Dragon Golem

Ice Walk. The dragon can move across and climb icy surfaces without needing to make an ability check. Additionally, Difficult Terrain composed of ice or snow doesn’t cost it extra movement.

Legendary Resistance (4/Day, or 5/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Aversion to Fire. If the golem takes Fire damage, it has Disadvantage on attack rolls and ability checks until the end of its next turn.

Berserk. Whenever the golem starts its turn Bloodied, roll 1d6. On a 6, the golem goes berserk. On each of its turns while berserk, the golem attacks the nearest creature it can see. If no creature is near enough to move to and attack, the golem attacks an object. Once the golem goes berserk, it remains so until it is destroyed or it is no longer Bloodied.

The golem’s creator, if within 60 feet of the berserk golem, can try to calm it by taking an action to make a DC 15 Charisma (Persuasion) check; the golem must be able to hear its creator. If this check succeeds, the golem ceases being berserk until the start of its next turn, at which point it resumes rolling for the Berserk trait again if it is still Bloodied.

Immutable Form. The golem can’t shape-shift.

Lightning Absorption. Whenever the golem is subjected to Lightning damage, it regains a number of Hit Points equal to the Lightning damage dealt.

Magic Resistance. The golem has Advantage on saving throws against spells and other magical effects.

Multiattack. The dragon makes three Rend attacks.

Rend. Melee Attack Roll: +14, reach 15 ft. Hit: 17 (2d8 + 8) Slashing damage plus 7 (2d6) Cold damage.

Cold Breath (Recharge 5–6). Constitution Saving Throw: DC 22, each creature in a 90-foot Cone. Failure: 63 (14d8) Cold damage. Success: Half damage.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Freezing Burst. Constitution Saving Throw: DC 20, each creature in a 30-foot-radius Sphere centered on a point the dragon can see within 120 feet. Failure: 14 (4d6) Cold damage, and the target’s Speed is 0 until the end of the target’s next turn. Failure or Success: The dragon can’t take this action again until the start of its next turn.

Frightful Presence. The dragon casts Fear, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 18). The dragon can’t take this action again until the start of its next turn.

Pounce. The dragon moves up to half its Speed, and it makes one Rend attack.