Mark of the Everglacier Monarch (Patreon)

Wondrous item, legendary (requires attunement by a spellcaster)

This wide necklace is adorned with forever-cold icicles that hang from it. While wearing the necklace, your skin develops a resilient layer of frost, granting you immunity to cold damage and resistance to slashing damage.

When you cast a spell that deals either acid, fire, lightning, poison, or thunder damage, you can choose to change the spell's damage type to cold instead. When you deal 35 or more cold damage to a creature with a single spell, that creature must also make a DC 15 Constitution saving throw. On a failed save, the creature is restrained by ice and must make another Constitution saving throw at the end of each of its turns. If it successfully saves against this effect three times, the effect ends. If it fails its saves three times, it is turned to ice and subjected to the petrified condition for 1 hour. If a restrained or petrified creature takes 10 or more fire damage (ignoring resistance and immunity), the effect ends early.

Curse. This necklace is cursed, and attuning to it extends that curse to you until you're targeted by a greater restoration spell or similar magic. Removing the necklace fails to end the curse. While cursed, your icy skin can splinter painfully when hit in certain ways: you are vulnerable to bludgeoning damage. If you die while cursed in this way, your body shatters into icy shrapnel. Each creature within 30 feet of you must make a DC 15 Dexterity saving throw. A creature takes 8d6 cold damage on a failed save, or half as much damage on a successful one. Your body leaves behind only a shimmering pile of snow-like dust, and you can only be brought back to life with a resurrection, true resurrection, or wish spell.

The cold throne of the everlasting ice was coveted as much as shunned, for the burden of the mark weighed heavy even on the soul.