"Night Reign" is set in the city of Laefendport, which is continously beset by magical, black rain that withers vegetation and drives animals mad, the "Night Rain". One of the local noble houses has discovered a process through which the Night Rain can be refined into a fuel, boosting their fortunes and those of their allied houses. Using their new-found wealth, the noble houses murdered the royal family in a power-grab. The players take on the role of former secret police agents who seek to eliminate the traitorous noble houses and to re-instate the monarchy, using stealth, guile, violence and devilry.
Each character has a Cover Identity, their everyday appearance to stay under the radar (though I'm not sure "Death Cultist" is an accepted occupation in this setting) which gives two talents and a set of contacts to use. Players have 13 additional points to distribute amongst the various talents, which are categorised as Stealth, Guile, Violence or Devilry. Each point in a Devilry talent will also give the character access to one 'prayer' (a magic spell). Up to five items may be selected as equipment, and that rounds out your character.

The meat of the game is in the incursions, when the group of agents attempts to achieve a specific overall goal. Each incursion is broken up in sets, each with their own goal that leads up to the conclusion of the incursion.
The preferred mode to achieve the goal is through stealth. The GM sets the scene and the players can describe how they go about their infiltration. The action resolution is through a blackjack mechanic: players draw cards to get as close to 21 as they can, and on a success (18-21) they gain a Shadow Token, which they can use to offset chaos and suspicion later on. Three Shadow Tokens can be used to reach the goal of the current set, but smart groups will try to get as many Shadow Tokens before moving on.
There are several additional mechanics to the blackjack resolution: a player can mark a piece of equipment or a talent to discard a card from their initial hand and draw a new one, increasing their odds.
But the biggest difference with 'regular' blackjack is that all of the 'face cards' are removed from the deck the players use: these are set apart as the Sister's Deck. The Sisters are the divine beings in the setting, but they are far from saintly! Each Sister has their own (unwholesome) domain with its own spells. If the character casts such a spell, they can take a face card from the Sister's Deck, increasing their chances of a good draw. But if the card they draw is associated with the Sister whose spell they are casting, they earn their wrath and have to take an (unwholesome) affliction...
The cards that the players draw are discarded face-up, so smart card-counters definitely have an advantage in this game. And unlike in regular blackjack, there are no rules against that!

If a stealth draw fails, then the characters may attempt to use guile to bypass any opposition they encounter through deception or fasttalking (basically everything except open violence) -- the lower the result of their stealth draw, the more opposition they find blocking their path. Using guile consumes Shadow Tokens, and the GM determines the risk associated with the player's proposed action. When resolving a guile action, the GM draws two cards of their own deck as the score to beat, with riskier actions meaning that one or even both cards are closed... Riskier actions have bigger rewards, but failure will get worse escalations.

If guile fails, there is always violence! The GM draws as many cards from their deck as the 'rating' of the opposition, and the player draws as many cards as their rating in the talent they use. Both sides select a card and describe what they are doing, and then both cards are flipped over. Black cards are defensive actions, red cards offensive. In case of both sides playing red, both hit -- but if one side plays black and the other red, then they score a hit only if the value of the red card is higher than the value of the black card. As the NPCs get hit, the GM discards cards from their hand -- and the opposition is defeated when there are no more cards left.
Letting a fight drag out nets the group Flame Tokens, which signify chaos and alertness during the overall incursion!

Once the incursion is finished, the group can use any left-over Shadow Tokens to neutralise Flame Tokens one-for-one. Any Flame Tokens left mean that your actions have drawn attention and the chaos in the city increases, making subsequent incursions harder. But successful exploits net you bubbles on the conspiracy track, which measures how close you are to fulfilling your goal of overthrowing the noble houses in the city.
A sample incursion rounds out the book, in which the characters infiltrates a wedding party in an Alcatraz-like fort on an island in the bay to assassinate the father of the bride!

The book is illuminated with a few pieces of evocative grey-scale art that help set the tone. But there's little additional information on the city or the rest of the environment, which can be a hindrance to GMs having to come up with suitable scenarios and opposition. On the other hand that leaves room for personal preferences. I would have liked to see a map with a rough description of each area (like in Blades in the Dark), just to get me started in visualising the environment.
The mechanics are certainly interesting. It's the perfect way to create a "push your luck" ruleset. I especially liked the way the magic spells work: they can give you a tremendous advantage, but there is a very real risk of an enormous drawback! One worry I have is that a group can never have too many Shadow Tokens, and as such the game incentivizes them to keep skulking around at the start of each set to generate those tokens. Instead of moving quickly towards their goal, the characters could spend a lot of time simply waffling about in order to get as many tokens as they can, which could potentially bog down the game.
If you want to know more about the ruleset and maybe want to use it in your own game of stealth and infiltration, then you should check out the 'Ruled by Night' SRD, which is available on itch (and oddly enough not on the website of the publisher, which is the web address shown in the logo of the SRD...).

Disclosure: I received a free copy of the PDF to review it from the publisher. The publisher did not have any influence on the text of this review, and this review reflects my personal opinion.