Chaos of ravens
The chaos of ravens is the name generally given to the spell employed solely by the Raven King, John Uskglass, to create servants for himself, to signify his presence, to awe and astonish his enemies, and for other, less understood reasons.
The spell causes thousands of ravens to appear, either transformed from nearby objects or created from thin air. The ravens may be performing a specific task, or simply be flying around confusedly. The chaos of ravens was seen on many occasions when the King was present or practising magic. It was seen by, among many others, the entire English army at the Battle of Newark, the Newcastle glovemaker's daughter and latterly of course by Jonathan Strange and Gilbert Norrell.
Strange and Norrell experienced the chaos of ravens in February 1817, when they boldly attempted to summon the Raven King to Hurtfew Abbey for the purpose of rescuing Arabella Strange from the gentleman with the thistle-down hair. As they performed their summoning spell:
"The candle guttered and went out...
...and in that moment...
...in that moment the room was full of ravens. Black wings filled the air like great hands gesturing, filled Strange's vision like a tumult of black flames. He was struck at from every side by wings and claws. The cawing and the croaking were deafening. Ravens battered walls, battered windows, battered Strange himself. He covered his head with his hands and fell to the floor." 
Although the chaos of ravens is a hallmark of John Uskglass's magic, it appears the gentleman with the thistle-down hair is able to call upon something very like it. When he first expels the intruding Strange from Lost-hope he does so by a tripartite spell the first stage of which features "a flock of birds" which for a second fill the great hall and then vanish.