The Dark Army, also known as The Black Army are the creations of Ravenna. They are made of a glass-like material that shatters upon impact, but can also wound and kill, due to its sharpness.
They first appear when King Magnus leaves to confront them after hearing rumours of their presence in his kingdom. Upon striking them, they shatter, and in the process, wound and kill some of his men.
Their last appearance was when Ravenna created some from the broken glass that reform from the throne room roof to stop Eric, William and some of the Snow White's men from helping Snow White to defeat Ravenna. Only to shatter and not to reform once Snow White had stabbed Ravenna.
Ravenna does have an army that is human mostly under Finn's comand but it is unsure whether they are another kind of the Dark Army.