In a fortress older than human history, from a time of war between Giants and Dragons, stands the tomb of a forgotten king. The Frost Giant lord of Rime Fang Keep hasn't heard his name spoken for eons, and thus it's lost to even him. What grand magic he must have once wielded, that his home can curse even a white dragon's blood to freeze in it's veins. His remains were hidden in a stone carved figure overlooking the vast lands beneath his mountain, the stormy peaks unable to obscure his sight. The mortals always came for imagined treasure, and as the ice claimed their bodies, his undeath claimed their souls. 

Then, a different group was caught in the storm, champions, heroes of some irrelevant fate. This time... For the first time, they'd leave his halls alive. Oh he'll send out the rotting guards, activate what's left of the wards, unleash the zombie nightmares he must have crafted long ago. Appearances must be kept. When the challenges are bested, his sarcophagus opened, they'll squabble over the only item of visible value. His eye, the vessel of himself.

Power! Power to command ice so vicious it would lay waste to nations. How could they resist? In their dream's he'll whisper. In their soul, his frost will creep. Then one day it will be him waking up to greet the dawn, with new flesh and heart, beating with cold blood.

Attuning to the Eye. To attune to the eye, you must press it into your empty socket. The eye freezes itself into your head and remains there until you die. If the eye is ever removed, you die. Your skin becomes cold to the touch, warmth now a lost comfort.

Properties of the Eye. While you are attuned to the Hoarfrost Eye you gain the following benefits:

Icey Veins. You are Immune to Cold Damage and environments, no matter how harsh.

Consuming Cold. Any spell you cast may be converted to cold damage, killing a medium or larger sapient creature in this manner will restore one charge. Charges may be consumed to regain equal value of spell slots. 1 charge for a level 1 slot, 2 charges for a level 2 slot or 2 level 1 slots and so forth.

Truesight. You have Truesight out to 240 feet.

Spellcasting. The eye has 8 charges and regains 1d4 + 4 expended charges daily at dawn. You can cast a spell on the Hoarfrost Eye Spells table from the eye (save DC 18). The table indicates how many charges you must expend to cast the spell. Once per long rest, the first time you use the last charge, roll a D20. On a 1, your soul freezes over. The Nameless King then takes control of the body like a puppet. If that happens, you become an NPC under the DM’s control.

Arctic Conditions. Each spell on the list does cold damage, and the target has disadvantage on all their spell saves in freezing conditions. 

Notes: Set Base: Truesight, Immunity: Cold