Eve starts with this item by default, and has the following extra effects:
The item is activated at one full Red Heart or less.
Her damage multiplier increases from 0.75 to 1.0 when active.
The effect is removed after leaving a room with more than half a heart (more than one full heart for Eve).
The effect may not be immediately applied if Isaac has lost red hearts without taking damage; the effect will activate upon entering a new room.
The effect is always active for characters without Red Heart containers, such as ??? and The Lost, though other characters can also lose their heart containers through Devil deals or items like Guppy's Paw or Abaddon.
III - The Empress grants a similar effect to Whore of Babylon for the current room which does stack with Whore of Babylon.
Stacking The Empress on Whore of Babylon either via multiple cards or multiple activations of Blank Card increases damage by the number of uses squared, e.g. +1.5 * 2 * 2 total damage bonus for using the card on top of activated Whore of Babylon.
Synergies
Blood Rights / Razor Blade / IV Bag: As these items takes Red Hearts before other heart types, it can be use to get the Whore of Babylon activation's condition easier.
Interactions
Keeper: As Keeper cannot have half of a Coin Heart, this item has no effect on him.
Rotten Hearts: Counts as a full Red Heart for the purposes of this item, rendering it impossible to activate (except for Eve).
Whore of Babylon: No further effect when multiple copies are acquired.
Eve as she appears under the effect of Whore of Babylon.
The pop-up that appears whenever Whore of Babylon activates.
Trivia
Upon obtaining the Whore of Babylon effect, a popup displaying the text "What a horrible night to have a curse" appears on-screen. This can be disabled in the game options.
In the original The Binding of Isaac, Whore of Babylon gave a higher stat increase just for the room it was activated in, with +2.16 damage and +0.36 speed.
III - The Empress was also much stronger, with +3.375 damage and +0.45 speed.
The Whore of Babylon is an evil creature mentioned in the Book of Revelations.
Half Red Heart or less.
Eve starts with this item by default, and has the following extra effects:
Red Heart or less.
??? and 
The Lost, though other characters can also lose their heart containers through Devil deals or items like 
Guppy's Paw or 
Abaddon.
This also includes 
Tainted Jacob after being hit by 
Dark Esau.
III - The Empress grants a similar effect to Whore of Babylon for the current room which does stack with Whore of Babylon.
Blank Card increases damage by the number of uses squared, e.g. +1.5 * 2 * 2 total damage bonus for using the card on top of activated Whore of Babylon.
Blood Rights / 
Razor Blade / 
IV Bag: As these items takes 
Keeper: As Keeper cannot have half of a Coin Heart, this item has no effect on him.
Rotten Hearts: Counts as a full Red Heart for the purposes of this item, rendering it impossible to activate (except for 
Whore of Babylon: No further effect when multiple copies are acquired.
The Binding of Isaac: Rebirth
