Ele é o Rei dos Corvos.
Quem mais ele poderia ser? Um mago de grande poder - o suficiente para reviver os mortos sem ajuda das fadas - que se veste de preto e, como se observa na pergunta, parece com fotos do Rei Raven visto anteriormente. Não esqueça, também, que ele mudou a escrita no corpo de Vinculus. Vinculus era a personificação do livro do Rei dos Corvos; a escrita sobre ele foi a profecia do Rei dos Corvos - quem, senão o Rei dos Corvos, poderia mudá-lo para mostrar algo novo e diferente?
Sua descrição no livro torna sua identidade ainda mais clara:
The man wore black boots and a black travelling coat. He was half-stooping, half-kneeling on the snowy ground beside Vinculus. For a brief moment Childermass thought it was Strange –but this man was not quite so tall and was somewhat slighter in figure. His dark clothes were clearly expensive and looked fashionable. Yet his straight, dark hair was longer than any fashionable gentleman would have worn it; it gave him something of the look of a Methodist preacher or a Romantic poet. "I know him," thought Childermass. "He is a magician. I know him well. Why can I not think what his name is?"
Out loud he said, "The body is mine, sir! Leave it be!"
The man looked up. "Yours, John Childermass?" he said with a mildly ironic air, "I thought it was mine."
It was a curious thing but despite his clothes and his air of cool authority, his speech sounded uncouth –even to Childermass's ears. His accent was northern –of that there was no doubt –but Childermass did not recognize it. It might have been Northumbrian, but it was tinged with something else -the speech of the cold countries that lie over the North Sea and –which seemed more extraordinary still –there was more than a hint of French in his pronunciation.
[...]
"Does the land hereabouts belong to you, sir?" [Childermass] asked.
"Yes."
"And where is your horse? Where is your carriage? Where are your servants?"
"I have no horse, John Childermass. I have no carriage. And only one of my servants is here."
"Where?"
Without troubling to look up, the man raised his arm and pointed a thin, pale finger.
Childermass looked behind him in confusion. There was no one there. Just the wind blowing across the snowy tussocks. What did he mean? Was it the wind or the snow? He had heard of mediaeval magicians who claimed these and other natural forces as servants. Then comprehension dawned on him. "What? No, sir, you are mistaken! I am not your servant!"
"You boasted of it, not three days ago," said the man.
-- Jonathan Strange & Mr Norrell, Chapter 67 (emphasis mine)
Uma figura misteriosa com grande poder mágico e um sotaque estranho, que afirma possuir tanto o corpo de Vinculus quanto a terra circundante, e a quem Childermass se vangloria de servir - vamos lá, só há um homem que pode ser.
Mas ele não veio por causa da convocação de Strange e Norrell.
Você está certo que a tentativa de Strange e Norrell de convocar o Rei dos Corvos para eles falhou, e eles, em vez disso, convocaram Stephen Black, o novo 'Nameless Slave'. Mas ao pensar que eles são os que puxam as cordas, que eles ainda tinham o poder de afetar o Rei dos Corvos, você está cometendo o mesmo erro que Childermass fez no livro:
"So?" said Childermass, stung. "That is not so very trifling, is it? Norrell is a clever man - and Strange another. They have their faults, as other men do, but their achievements are still remarkable. Make no mistake; I am John Uskglass's man. Or would be, if he were here. But you must admit that the restoration of English magic is their work, not his."
"Their work!" scoffed Vinculus. "Theirs? Do you still not understand? They are the spell John Uskglass is doing. That is all they have ever been. And he is doing it now!"
-- Jonathan Strange & Mr Norrell, Chapter 67
(John Uskglass == o Rei Raven, é claro - não me lembro se esse nome foi mencionado na série de TV)
É o Rei do Corvo que está puxando as cordas, e tem sido desde o início. Foi ele, através de sua profecia e seu agente Vinculus, que trouxe Strange e Norrell juntos em primeiro lugar. Ele apareceu para ressuscitar Vinculus e reescrever a profecia porque ele queria, não porque eles tentaram convocá-lo.
Nos livros, Strange e Norrell realmente sabem que ele estava na Inglaterra, mesmo que nunca o tenham visto pessoalmente. Como você, eles cometeram o erro de acreditar que o feitiço de convocação deles tinha algo a ver com a presença dele lá:
Norrell did the magic, naming John Uskglass as the person they sought. He divided the surface of the water into quarters with lines of glittering light. He gave each quarter a name: Heaven, Hell, Earth and Faerie. Instantly a speck of bluish light shone in the quarter that represented Earth.
[...]
In a tone of wonder, Norrell said, "I think we may have succeeded after all! It says he is here. In Yorkshire!"
-- Jonathan Strange & Mr Norrell, Chapter 66
E mais tarde, quando ele retorna ao seu próprio reino, eles refletem sobre por que ele não veio a eles apesar de seu charme convocando, a razão pela qual se resume a "ele era muito orgulhoso":
In the silver dish of water the speck of light flickered and disappeared.
[...] "He is gone," he said. [...] "It is very odd," continued Norrell, in a tone of wonder. "What do you suppose he was doing in Yorkshire?"
"Oh!" cried Strange. "I dare say he came here on purpose to make me mad!" With a cry of mingled rage and self-pity he demanded, "Why will he not attend to me? After everything I have done, why does he not care enough to look at me? To speak to me?"
"He is an old magician and an old king," answered Norrell briefly. "Two things that are not easily impressed."
-- Jonathan Strange & Mr Norrell, Chapter 68