Having not read Black Easter/The Day After Judgement by James Blish, the book that inspired Tony Boydell to create Aleph Null, I am perhaps missing the references that the game makes to its sources. The Grimoire Verum (the 'true magical textbook' that includes the rules and the Book of Hours - the game's timer) says that an eccentric rich gentleman approached us, a student of the occult, and asked us to summon the arch-demon Baphomet merely on a whim: 'Turn it loose in the world, with no orders and no restrictions, and see just what it would do. As an aesthete of destruction, my motives are purely artistic'. One can picture the amoralist aesthete Des Esseintes, the protagonist of Huysmans' Against the Grain, commissioning a young Aleister Crowley to perform the dark ritual. Since these activities sometimes involve sex magic, Mr. Boydell conveniently included a penis tree.*
So, how does one summon a demon? The instructions are simple, but the process will require familiarity with the cards before you can fully succeed. This is a deck destruction game, in which, round after round, you are sacrificing cards from the play area, your hand and the discard pile, because Baphomet dislikes clutter. At the end of the ritual, only three cards must remain in play: the Book of Pacts, the Wand of Power, and the Grand Circle. If more cards linger in the play area, they deduct from your score. If you still have cards in your hand or the discard pile, the game is lost.
At the start of each turn, you draw a hand of 5 cards. Most of them have a cost in magical power but your trusty Tanists (your helpers) and some candles are free and can be 'scrapped' so that you can start amassing your tools. There are several useful cards in the deck which, if sacrificed at the right time, will help you get rid of unwanted artifacts and most importantly, 'interferences'. Interferences will be added to your deck after the end of the second hour (if you play on easy/standard) or at the end of each hour (if you play on advanced difficulty). As the name implies, they are meant to interfere with your work, causing you, for example, to discard your hand or preventing you from gaining sacrifice benefits.
The most satisfying aspect of the gameplay is the formation of combos. These will be revealed to you the more you play the game and grow accustomed to the abilities of the cards. The Mirror card, for instance, may come in handy when you decide to get rid of two cards instead of one, or when you sacrifice, let's say, a salamander, and mirror it to get extra magical power. You can ring the Bell to summon an expensive card for free, and use the Exorcised Water and the Weeping Tree to clear out a card from your hand or discard pile.
Time is of the essence in this game. Not only the limited time you have to complete the conjuration, but also the time you choose to make a decision. Some cards are best left to be brought in play in later rounds, and others should be present early on.