(How) Can PCs use the frosty orb of a one-eyed shiver?
The players in a Princes of the Apocalypse campaign I'm DMing have just defeated Morbeoth, a one-eyed shiver in the Crushing Wave cult. One player saw the magical orb he had in his eye socket and immediately wanted it for himself.
Other cultists equipment sometimes contains caveats in the description preventing players from taking or using it, such as the Black Earth guards who's devotion to their Elemental Prince allows them to wear their special plate armor which breaks down when they're slain. Morbeoth's eye seems to offer no such restriction.
From their description in Princes of the Apocalypse, page 207:
A one-eyed shiver is a fearsome cultist who wields powers of ice and cold. To gain the frigid power of elemental water, the shiver removes one eye and replaces it with a frosty white orb that can blast foes with an icy ray. When the orb isn’t in use, the shiver covers its magical eye with an eye patch. Even when hidden, the orb’s magical power makes itself felt—a one-eyed shiver has ice-cold blood and is constantly surrounded by an aura of thin, cold fog.
It seems to be the source of their Chilling Mist trait and Eye of Frost action.
I'm currently considering allowing it, contingent upon finding a reasonable way to balance the powerful abilities it seems to grant. I'd like it to be an option for the players, but I don't want it be a "must-have" power boost. The PCs just gained enough experience to level up during the session so dipping into another class to meet prerequisites isn't a major hurdle at the moment. While this applies especially to a particular player, I'd like to be able to apply any ruling I make consistently. The item in question isn't unique in the campaign, or even the current dungeon. If I offer an option to a ranged attack focused rogue or bard, that reasonably balances the item so they don't significantly outperform the rest of the party, and then they decide not to take it, I don't want to have to move the goalposts in order to make sure it's still not game breaking if a melee focused paladin or fighter wants to take it instead.
For additional context, the one-eyed shiver is a human 5th-level spellcaster, who uses sorcerer spells. The Chilling Mist trait creates a ten foot aura that causes creatures to take 1d10 extra cold damage when the shiver damages them. The Eye of Frost action casts ray of frost and restrains the target on a hit, until they use an action and succeed on a DC 13 Strength check. (The DC may be based on the Charisma or spell save DC of the user.)
Would it be reasonable to allow a player character to use the frosty white orb at all in order to gain these abilities? Or is it not appropriate to allow a player access to these powers?
To mitigate the additional close range damage buff, would it be reasonable to require attunement, or attunement by a spellcaster, or sorcerer specifically?
To mitigate the damage dealing/powerful debuff cantrip, would it be reasonable to require the player to sustain the effect of the Lose an Eye. entry in the Lingering Injuries table on page 272 of the 5e DMG?
Is there another feasible way to handle this, some other corrective measure to deal with the effects, or a combination of other options? Am I underestimating the potency of the abilities, or is there other balance issues I haven't considered?