Civilization VI Secret Societies is a game mode that came out with the Ethiopia Pack. It introduced the concept of Secret Societies in the form of governors whose promotions give us powerful bonuses, access to new buildings, units and improvements. We will focus on The Voidsingers strategies and perks.
The Voidsingers are a nihilistic cult of dark elder gods. They focus on religion, relics and loyalty.
Reach the Medieval Era
Cities earn Gold, Culture and Science per turn equal to 20% of their Faith per turn rate.
Reach the Industrial Era
Unlocks the Cultist unit. This unit is purchased with Faith and uses charges to reduce Loyalty in foreign cities and generate Relics of the Void.
Reach the Atomic Era
Unlocks Dark Summoning, a city project that provides lots of Faith while active, and upon completion, raises the amount of Loyalty damage done by Cultists.
+4 extra Faith in every city is a huge bonus, to put this into perspective, having an Old God Obelisk provides the same Faith as a Temple, which is double the production cost and its other bonuses/perks are comparable to that of the Old God Obelisk. Civilizations that leverage Faith well into other yields, units or great people benefit the more from the building.
Ethiopia led by Menelik II converts 15% of Faith into Science and Culture in cities that have been founded on hills. This means Old God Obelisks also provides +0.8 Science and Culture (which the game rounds up to 1). The bonus goes even further with the Voidsingers second promotion level, which also converts Faith into Culture, Science and Gold too.
Malians benefit a lot from the extra Faith too, being a Civilization capable of buying Commercial Hub buildings with Faith.
For every Civilization Old God Obelisks can make the difference when competing for Prophets, buying them out with Faith and therefore being capable of founding a religion without neglecting Civics tree advancement or early war.
The Cultist is a support unique exclusive to the Voidsingers. It is used to damage loyalty in foreign cities and generate Relics.
The Voidsingers work well with Civilizations centered in Religion and Culture. This types of victories should be your goal, wars will be mostly defensive and the way of expanding your territory is going to be through settlers and by flipping foreign cities using loyalty.
Discovering a tribal village grants a 70% chance of meeting the Voidsingers, so make sure to explore if you're playing a religious/cultural civilization. It might take some tries as chances lower when other Civilizations discover them first.
Voidsingers give you access to Old God Obelisk, a powerful Monument replacement that will not only grant its usual Culture and loyalty, but also +4 Faith. Use this to buy out a Prophet if you're too late in the makings of Holy Sites and Shrines.
Having Old God Obelisks in your first 5 cities will benefit you immensely in the later eras of the game, for instance, in 15 turns you will produce 300 Faith, which will make the difference to snatch pivoting Great Persons.
The Medieval Era is when you thank your younger self for building enough Old God Obelisks and religious buildings in general, as thanks to the Voidsingers second promotion 20% of the Faith output will be converted into Science, Gold and Culture. This will push your Science and Civics tree forward helping you leave the AI behind.
Buy Great Persons with Faith and start making sure your citizens are happy (trade luxury resources with other Civilizations) and the population is growing. Happiness and population will play in your favor when it comes to loyalty and flipping cities, which is the next stage of the game.
In the Industrial Era you gain access to the Cultist, a unit that is purchased with Faith, and uses charges to reduce loyalty in foreign cities. If you played the previous parts correctly you will have enough Faith to purchase an army of Cultists, and send them to 'conquer' foreign cities. This coupled with your cities loyalty will make it so you will be able to flip a lot of cities.
Place Amani (Governor) in a city-state state close to foreign cities, use the Emissary (Cities within 9 tiles have +2 Loyalty per turn towards your civilization) and Prestige (Your own cities within 9 tiles have +2 Loyalty towards your civilization).
In the Atomic Era you gain access to Dark Summoning, a city project that provides Faith while active, and upon completion, raises the amount of loyalty damage done by Cultists. Keep flipping cities, although this will enrage other Civilizations so prepare for a defensive war. Artillery defending your cities and a technology advantage should be enough to waste their time while not harming your religious or cultural victory.
Ethiopia is one of the civilizations that have best synergy with the Voidsingers. The second promotion (20% of Faith you will gain as Science, Culture and Gold) works really well with Menelik II's ability, Council of Ministers (Cities founded on Hills gain 15% of their Faith output as bonus Science and Culture). These two bonuses together make up for a 35% Faith to Science and Culture bonus, which will skyrocket your advancement in the science and civics trees. Having Tanks against the AI Cavalry will not only be usual, but also unnecessary, since you will be achieving a Cultural or Religious victory already.
The english led by Eleanor of Aquitaine are not particularly religious, but they are great at flipping cities through loyalty. In this regard the Voidsingers with their exclusive unit, the Cultists, will give you a boost that will make it even easier conquering the world without wars.
Russia is a religious civilization that has a replacement for Holy Sites, the Lavra provides 1 extra Great Writer, Prophet, Artist and Musician points. Additionally Russia gains extra Faith and Production from trunda tiles, which works well with the Voidsingers second promotion.