So, after weeks of wading through Shadowfell, my party of soon-to-be-13th level adventurers have reached the Raven Queen to ask her some very important questions about the plot, which is all fine and dandy. HOWEVER, the sorcerer has given very strong hints that he wants to make some sort of deal with her. Now, this could just mean he wants to multi-class into warlock, but I would also like to give him an option that does not force him to multi-class, yet still allows him to have that special connection.

Now, the game is pretty high magic, and they all have their fair share of uncommon and rare magic items, But I still would not like to give him something that makes him completely unbalanced.  The current idea is an adapted Shadowfell Shard:

- Requires attunement

- After you cast a spell using a Metamagic option while you are attuned to the shard, you can momentarily curse one creature targeted by the spell; choose one ability score, and until the end of your next turn, the creature has disadvantage on ability checks and saving throws that use that ability. 

- You gain Devil's Sight to a range of 60ft

- You no longer needs to pass a test to be revived (we use an adaptation of Matt Mercer's revival rules), but the spell costs double material components (they have more than enough money to get diamonds now, so this isn't a huge issue for them)

- No unatuning without anything short of a wish or divine intervention (as it will be attached directly to his soul)

The changes to the Shard's effect would make it take effect after the spell is cast (so they can't give disadvantage to the saving throw of the spell they are currently casting), to try and make it less overpowered than it already is. The sorcerer already has a Bloodwell Vial +2 (yeah, probably shouldn't have given that to him in hindsight), and this forces him to have to plan ahead to his next turn. 

The Devil's Sight gives him a warlock-ish feel, and the other two are the cost of his bargain (along with roleplay-derived issues of making such deal and having a paladin in the party). It feels overpowered to me, but mostly because the normal workings of the Shard already feel overpowered, yet at the same time I want it to feel like a hefty boon. Anyone has any suggestions on balancing, or in better effects that would give it a more death-related feel?

At level 13, I don't see this as too powerful.  For one, the attunement cost is 1/3 of his potential resources, so it needs to be worth it at that level.  Devil's sight of any kind is a powerful boon.  The metamagic option does require preplanning, so I like that.  The revival test is cool and I like it that it will definitely sway the character into attuning to the item.  Just make sure some of the cost isn't about the mechanical aspects.  Have the player visited by haunting dreams.  Have him forget some memories from time to time (as the Raven Queen plucks them from his mind) or other kinds of shadow-manifested dealings.  Have her make a request as part of the bargain.  There's all kinds of ways to backweight this kind of a deal if the player attunes to it.  You just have to entice them enough to make the bargain... and then, over time, they get to begin to see the full scope of the writing on the wall.

Yeah, there will be both known and unknown roleplay aspects of him accepting the deal.

For knowns, some task for him to fulfill (not quite sure what yet). For unknown, he has recently made a bargain with a creature, trading a piece of his mind for an items and lost the memories of his dead wife in the process. He still knows he lost someone, he just can't remember what she looks like or his relationship to her (though he has some notion). Why, I find it only fair that she decides to fill the gaps with her own image :D

You could have him switch his subclass to shadow sorcerer, as the raven queen's magic seeps into his veins. 

There is an old UA for a Raven Queen warlock that was actually kinda cool.
http://dnd5e.*******.com/warlock:raven-queen-ua

Maybe you could give a little taste of that sentinel raven level 1 ability and the option to multiclass into a Pact of the Raven Queen warlock?

Alternative if he wanted to multi class to paladin or cleric with her as his patron goddess. 

I'd consider changing the metamagic effect to apply to checks and attacks rather than checks and saves. Disadvantage to a save of your choice - especially with no chance to resist it - is very strong when combined with some of the higher level save-or-suck spells (and multiply this effect by how many spellcasters are in the party). It will absolutely be used on every boss enemy, and has the potential to trivialize those fights (or to make you work that much harder to create a boss fight that can survive this).