It's a  black, hooded cloak with long coattails resembling bat's wings, hence the name. Anyone who wears this cloak can easily move at night without being noticed. It grants a bonus +5 to all Dexterity (Stealth) checks performed at night in dim or with no light. 

For those familiar with arcane arts this item reveals far more superior features. Entwined in its weave, there is powerful spell nullifying any necrotic damage dealt by spell of level 3 or lower. This feature can be activated once. After that the user needs to perform a recharging ritual. The description of the ritual can be found in a tome called "Conflagration" or be acquired from any Orcus related cult member, clergy or demon. 

Conflagration, Book of The Bat, 4, 4-9

'[4]...those who are willing to serve our Lord, and who do not quiver when the death ask for their name, will be granted attributes and powers and means to withstand the dangers and struck the enemies. [5] And to be like a bat at night, like a dagger that strikes without a warning, He gave us mantles and cloaks. Now we shall be the demise of those who doubt. [6] Do not steal the cloak of others or the punishment will be great and He will bring pain and ten times the death you would suffer from His enemies. [7] The chosen will use 'em for far greater purposes.[8] When struck by arcane that He is a master of, you will not suffer, I promise you. Do not fear of those who try to struck you, because you are a chosen.[9] And when the blessing of the Bat will come, you shall take the mantle off, put it on the ground and pray to the Lord with this words 

Nothing shall struck me 
The death is within 
Let the Bat devour 
What's His

and the blood will be spilled, both yours and other creature so that the Bat, our Lord knows, your love and devotion is real.'

Notes: Bonus: Stealth, Meditation and spilling blood, Buff, Outerwear