Charlie

Charlie, otherwise known as the Night Monster, or (unofficially) the Grue, is an invisible hostile damage source. She only attacks when the character is in complete darkness, unable to see. (Characters will state when they are vulnerable, e.g. "It's so dark!"). When being stalked, the character will become alarmed, exclaiming things such as "What was that?", followed by a loud "whoosh" to signal her approach. Moments later, a slashing sound is heard and the character will take 100 damage and 20 Sanity loss. These attacks will continue every few seconds until the character finds or creates a light source in order to see.

Most characters can survive a single attack, even without armor.

Not much is known about Charlie, but what is known is she was the female assistant of Maxwell when he was a magician before the main events of the game's story. Her gender was first revealed by Kevin (lead programmer) on the 'Caves' livestream.

Light Sources
The following light sources will prevent Charlie from attacking:
 * Inventory/equipped items:
 * A Heat Stone at full charge
 * A Lantern while equipped or dropped (if not turned off)
 * A Light Bulb (on the ground)
 * A Glow Berry (dropped)
 * A Miner Hat (while equipped)
 * A Pumpkin Lantern (on the ground)
 * Star Caller's Staff's Star
 * A Torch (while equipped)
 * Willow's Lighter (when equipped)
 * A Magiluminescence (while equipped)
 * The force field effect of a Thulecite Crown (once activated)
 * Structures or world objects:
 * A cooking Crock Pot (character must stand in front of it)
 * Fire (Campfire, Fire Pit, Willow-made fires, or burning objects)
 * A Light Flower (when not picked)
 * A charged Lightning Rod
 * A Mushtree (intact)
 * A Night Light (with fuel)
 * A Pig Torch (with fuel)
 * A Walrus Camp's light when a member of the MacTusk N' Son Hunting Party is occupying it (during winter)
 * A Nightmare Light or Nightmare Fissure while active
 * An Ancient Pseudoscience Station (while near)
 * Maxwell's Light (while near)
 * Characters and mobs:
 * Abigail's glow
 * Fireflies' illumination (coming close to them will cause them to disperse. If players walk away, they will come back almost immediately, making it possible to "hop" between two or more patches of fireflies and survive the night. It can even be done with just one patch, walking in and out of repeatedly, although this is more difficult and the player suffers greater sanity loss)
 * A Ghost's glow
 * Slurper (while it's awake and/or on the character's head)
 * WX-78 on OVERLOAD after being struck by lightning
 * WX-78's sparks (when it's raining)
 * Any mob or character that has eaten a Glow Berry
 * A Depths Worm's displayed lure
 * A Houndius Shootius when charging a shot.

Trivia

 * The nickname Grue is a reference to the iconic monster from the game Zork. In Zork, the Grue only attacked players in pitch darkness after a line of warning, just like in Don't Starve.
 * Since Underground, Maxwell refers to the Night Monster as Charlie.
 * Woodie in Werebeaver form has night vision, and is not attacked by Charlie. This suggests that Charlie is not averse to light itself, but rather the ability to perceive her.
 * Charlie was Maxwell's assistant and it is speculated that she was transformed in an accident during a ritual.
 * When attacked by Charlie while playing as Maxwell, he will say "Charlie? It's me! Maxwell!"
 * When killed by the Grue, the cause of death listed in the Morgue is listed as "Darkness", except for Maxwell, whose is listed as "Charlie".
 * On the picture "Charlie" is holding a book. It is believed that the book is the "Codex Umbra ".

Bugs

 * If a player enters a Worm Hole just before the night sets, Charlie will not attack when the character comes out of the other side. (This bug is stopped when going into light and then back into darkness again.)
 * If the player pauses the game when the sound of Charlie attacking begins and resumes right before the sound ends, Charlie will attack early.