Monster deck

The Monster Deck contains monsters, curses, and bonus cards. There are 100 monster cards in the base game, and an extra 20 in the gold box expansion.

Gameplay
The game begins with two active monster slots. During your turn, you may choose to attack one of the available monsters in play, or attack the top card in the Monster Deck, putting that card on top of one of the available active slots. Then, you must keep attacking until either you or the monster dies. When you kill a monster, you are rewarded with the rewards on the card.

There are 4 aspects to any monster:

Health: The amount of health the monster has

Dice Roll: The roll you have to get to damage the monster, when you get that roll or more, you will damage that monster equal to the damage shown on your player card (including items or anything that boost your damage)

Damage to Player: The amount of damage that's done to the player if the player misses (not rolling a high enough number)

Rewards: When the Monster is killed, the reward the player gets

Basic Monsters
These monsters are typically weaker than boss monsters, and thus they usually have smaller rewards.

Holy/Cursed Monsters
Holy/Cursed monsters are monsters with special effects that can be good or bad. These effects usually only take effect when a certain number is rolled. They can be identified with a white or purple background respectively.

Bosses
Strong enemies that always yield one soul upon defeat. The only exception to this is Delirium, who instead allows a player to gain two treasure cards upon being defeated.

Mega Bosses
Mega Bosses are incredibly hard-to-fight versions of bosses, of which only a few appear in the game. There are 4 of these in the game, Hush being Gold Box exclusive. These Mega bosses include: Mom!, Mom's Heart, Isaac!, Satan!, Hush, and The Lamb. They are high risk, and high reward Bosses to fight. They all yield two souls (although The Lamb is an exception, as you gain one soul from him, but also steal one from another player).

Bonus Cards
Instead of fighting these cards, they enact special effects, and sometimes yield rewards.

Curses
Curses are negative effects that last until you die.

Additional notes

 * Delirium doesn't fit anywhere on the list, as said in a tweet by Edmund, yet still acts like a monster card, and is in the monster deck.
 * If a card has has "Draw #" or "Loot #'' on it's reward, it's always referring to Loot Cards
 * Holy and Cursed Monsters were original to the card game.
 * The only curse which already existed ingame was Curse of the Blind