Actually, Arsenov, you are wrong.
The one who summoned Turel to the depths of Avernus was not the Hylden; it was actually Azimuth the Planar from the first game. She was the Guardian of Dimension and was able to summon all manner of creatures from different moments in time, and from other dimensions. Hence the name. When she was corrupted like the other guardians, she and one other guardian (name and which guardian escapes me at the moment) created Dark Eden. A place that is without a doubt a very weird place. As Kain himself comments on when he's closing in on Dark Eden and Avernus Cathedral. There she proceeded to summon the creatures so named; the red demons that you see in BO1 is actually the precursor to the Hylden race, although, of course, we did not know that at the time.
Anyway, once Turel -had- been summoned—Azimuth didn't actually have any control over her magic, due to the corruption, and creatures of other times and dimensions would appear at will, including Turel—he was possessed by the Hylden who took upon the mantle of Hash'ak'gik, an apparent god, and with it used it to control the disciples of Avernus to give him blood sacrifices - in the form of every first born. And if they did not obey, they would "know his wrath." Obviously that was just a ploy by the Hylden as they only needed the blood [of the first borns] to keep the host body [Turel] alive. He was also in a pit, unable to get out (while being blind as a bat [literally]). Whether he had been summoned there to begin with, or if he had been thrown in by the people of Avernus, I do not know, and there is no evidence for neither.
As for 'Unspoken', it was the name given to the Hylden by the Ancients -- now known as vampires -- after being victorious in the battle between the two races and after the Pillars had been erected, banishing the Hylden race from the land forever.