Black loves the Faustian bargain. It is the color that can offer you the most power, but never without taking something from you first. Power will cost you; will you pay the price? Black often has you exchanging life or permanents for that promise of power. My favorite cards are those that provide great in-game mechanics while also evoking this theme of gain through sacrifice.
Some of my favorite cards are what I think of as "Faustian agents":
Each of these creatures offers you a "bargain": cards in exchange for life, energy, your creatures. Dark Confidant is a classic card of this type, but Glint-Sleeve Siphoner and Grim Haruspex also have been strong cards in their time. Our Guilds of Ravnica preview card joins their ranks; will it fall short of this bar, or will it rise to join this elite club?
The first thing I have to say: Midnight Reaper looks creepy.
Note how everything on the card is murky and gray, except for its scythe. The scythe sheens with a blue-green luminescence, ready to cut deeply into any who foolishly refuse to pay tribute.
Is it as scary in a game of Magic, however?
The first thing we can do is look at its rate. A 3/2 for 2B is not the scariest creature you have ever seen. For the same converted mana cost, Goblin Chainwhirler beats it up every time. 2 toughness makes it easy to Shock and clear from the table, and many two-mana creatures can trade up with it.
In terms of raw power, Midnight Reaper is no Steel Leaf Champion. Where we will need to evaluate it then, is the strength of its ability:
Whenever a nontoken creature you control dies, Midnight Reaper deals 1 damage to you and you draw a card.
How good is this ability?
First, let's note that Midnight Reaper, unlike Grim Haruspex, can trigger off its own death. The worst-case scenario with non-exiling removal is that the Reaper replaces itself when it dies. A card that replaces itself is always a good place to start.
If Midnight Reaper survives, it then creates instant card advantage from every creature that you cast or have already cast. If your opponent spends a card to remove a creature, you go replace it immediately. That bodes well for the type of attrition-based starts that most control decks tend to enjoy. Eventually, the control deck runs out of answers and you keep a steady supply of threats coming.
The only downside here is that pesky "1 damage to you" in the card text. Getting cards this way is not free, and over time, that small loss of life could add up, especially since the ability has no optional words like "may" in it; whenever you lose a creature, you have to take the card and the damage.
Overall, one can judge Midnight Reaper's ability to be pretty strong. The next question is, what decks might it fit in?
A Tale of Two Types
Every creature has a type, and almost every set of Magic these days has payoffs for different creature types. Midnight Reaper belongs to two tribes with relevant representation in Magic. A creature that is a Zombie Knight (the only other one we know of in Standard now besides Josu Vess, Lich Knight) has some options.
As a Zombie, Midnight Reaper pairs well with:
Isareth, Liliana, and Graveyard Marshal offer their own card engines that combine quite well with dying creatures. Will Midnight Reaper add enough card advantage that we will see another Zombie Uprising?
As a Knight, Midnight Reaper goes with:
Adding Midnight Reaper to a White-Black Knight deck is easy to imagine, but I do want to call out possible synergies with Paladin of Atonement. The Paladin and Midnight Reaper offer potential payouts to each other that might be worth considering. The Reaper can draw cards and also fuel the Paladin, while the Paladin grows and returns life lost from the Reaper when killed. Not the type of synergy to build your whole deck around, but something to consider.
In addition to what it does for its respective tribes, Midnight Reaper also adds value to sacrificed-based cards.
Sacrificing creatures with Reaper on board gives them the extra ability "take 1 damage, draw a card" in addition to whatever else they are doing. You will have to do something to mitigate the life loss, but assuming that can be done, you have access to some lethal card engines and combos.
You Reap What You Sow
In general, a lot of what I think makes this card a solid play is that it brings overall value to any deck with creatures. Playing Midnight Reaper will sometimes force hard decisions. Kill the Midnight Reaper now (knowing its owner gets to draw a card for it immediately) or hold removal for bigger threats and allow Reaper's owner to draw more cards?
I think that the Midnight Reaper represents such overall value that any creature-dense aggro or midrange deck will want to at least consider it. The damage it inflicts is worth noting, as it leaves you susceptible in damage races or to direct damage