The Candle is an activated item in the Wrath of the Lamb DLC for the original game, and the Rebirth remake. It is a candle on a stand, emitting a blue flame.
Upon activation, Isaac will raise the candle above his head, causing his movement Speed to reduce. On pressing a fire button, a blue flame projectile will fire in the chosen direction that deals significant damage to enemies that it comes in contact with. Enemy shots will also be destroyed on contact.
The flame projectile can pass over Pits, but can not travel through Rocks or Poop.
Combined with The Battery, The Candle will charge every 1.5 seconds, with the 9 Volt, every 1.2 seconds and combined with both will make The Candle recharge every second.
The Candle inadvertently synergizes well with the Bloody Lust and Samson's Lock items. Whenever it destroys enemy tears, it counts as a kill, which in turn grants Isaac increased Damage.
The flame travels a reasonably short distance - about 5 spaces, the same as a Dr. Fetus bomb shot.
The flame will bounce off of rocks and walls, also like a bomb shot. This can be utilized to get additional hits on high HP enemies via a rebound.
If Isaac's movement speed is too high to easily maneuver around hazards, The Candle can be used to slow him down to a more manageable Speed.
This item is a reference to the Blue Candle item from The Legend of Zelda, one of the games that inspired The Binding of Isaac.
The Candle can damage Mom when used on the doors in her Boss Room, Mom's Heart when it's not in view and Satan when he's still in his statue form.
The Candle can kill War and Conquest while they are off screen. Though the death sound can be heard, the health meter remains on screen and they never return. As a result, Isaac will be trapped in the Boss Room, with the only means of escape being teleportation items (Teleport!, Telepills, etc.).
Monstro can be damaged while in the air by hitting his shadow with the candle's flame.
The three upper bugs can be explained: The flame fired by The Candle damages enemies the same way a Fireplace would. Both ignore the fact that Isaac or enemies are flying or have their hitbox 'hidden'. In other words, they will always inflict damage (except Isaac are applied with effects such as The Gamekid, since it provides Isaac with invulnerability rather than hiding Isaac's hitbox.)
When combined with Technology 2 as Cain, The Candle cannot be fired at all. This does not apply to other characters.
While The Candle is active, other animations are negated such as pill and item pick-ups.
When firing The Candle either left or right, Isaac's hair will face the wrong direction for a short period of time. This is merely a cosmetic bug.
If Mom's Knife has been thrown and The Candle is activated, it causes the knife to hang in the air until The Candle has been fired or deselected. This interaction, when combined with homing tears (Spoon Bender, Sacred Heart, or The Magician), causes the floating Knife to home in on enemies. Said knife may only hit a given foe once for contact damage.