Rules Card is a special card which randomly displays cryptic statements on the screen similar to a Fortune Teller.
It is unlocked by completing Challenge #11, Glass Cannon.
Upon activation, displays one of the following messages:
CHUB DISLIKES SMOKE!
Referencing the trick in which Isaac can drop a bomb in front of a charging Chub to have her swallow it. Reference to Zelda 1 and 'DODONGO DISLIKES SMOKE'.
A PIECE OF PAPER IS YOUR GUIDE
Possibly refers to the or Missing Poster trinkets. Could also be referencing the Rules Card itself.
THE WALLS WILL HARDEN OVER TIME. TIME IS THE ESSENCE
Presumably referencing the Boss Rush that occurs if Mom is beaten in under 20 minutes.
A DARK MARKET LIES UNDER YOUR FEET
Presumably referencing the secret trapdoors occasionally under rocks and the rare trapdoor that leads Isaac to the Black Market, a devil room-like area.
SOME DOORS REQUIRE A BLESSING. CARRY THEM WITH YOU
Refers to how to open the large golden door on The Chest and the Dark Room, by gathering two Key Pieces from the Angels.
DENY HIS GIFTS TO ATTAIN YOUR REWARD
References how skipping a deal with the Devil gives the chance to gain access to the Angel Room.
SLEEPING GATEKEEPERS WILL NEED TO BE AWOKEN WITH A LOUD SOUND
Possibly referencing how the player can use bombs on the angel statues in order to awaken them. Reference to Zelda 1.
ROOMS MAY YIELD MORE THAN YOU EXPECT. EXPERIMENTATION IS KEY
Unknown, possibly referencing Secret Rooms or the trapdoor and bed in Isaac's room, or tinted rocks and other breakable objects.
One of the following
Seeds may also be displayed: