Blood Omen era
In Blood Omen, Ariel told Kain to "beware the Unspoken", as he was starting his quest to kill the corrupt Pillar Guardians. Throughout the course of that game, it became apparent that this malevolent force - alternatively called the Unspoken, Hash'ak'gik or the Dark Entity (the latter two were explicitly used by Silicon Knights in the Plot section of the Silicon Knights Blood Omen FAQ) - was responsible for possessing Mortanius. Under its control, the Necromancer had murdered Ariel.
The Dark Entity - as it is most accurately called, based on the later games - had known this act would attack all of the Guardians in one move, initiating the fall of the Pillars. Nupraptor, driven to insanity by the grief of his loss, turned his magic on the Circle, infecting them with his madness. Kain, destined to be Ariel's replacement as Balance Guardian, was tainted by Nupraptor's assault from birth.
Mortanius could occasionally free himself from the Dark Entity's influence. During one of these interludes, he had Kain murdered, then brought him back into the world as a vampire. Kain set out to purge the Circle of its corrupted members under the direction of the Death Guardian and the spectre of Ariel. At the end of Blood Omen, Kain killed Mortanius at the Pillars. As the Necromancer died, the Dark Entity emerged, deforming Mortanius' flesh into a demonic form, and taunting Kain for being its pawn. Kain protested that he served no-one, and he fought the Dark Entity, eventually beating it. However, despite outward appearances, he hadn't destroyed it. He had only destroyed the remains of Mortanius' body, which housed the Dark Entity's soul.
Blood Omen 2 era
The Sarafan Lord seemed like an entirely new enemy when he was introduced in Blood Omen 2. In fact - as Defiance and the first Defiance Q&A page at thelostworlds.net would show - he was simply the Dark Entity inhabiting his own, natural Hylden body.
In Blood Omen, Kain refused to sacrifice himself to repair the Pillar of Balance. This sealed the Pillars' fate, condemning them to collapse. The barrier between Nosgoth's Material Realm and the demon dimension weakened substantially, and a powerful Hylden - a leader amongst them - was able to re-enter the Material Realm using the Hylden Gate. He gradually began to increase his power in preparation for a full invasion. He established - or re-populated - a base (the Hylden City), and started to draw other Hylden into Nosgoth's Material Realm with him. However, it was not enough to bring his people through slowly - his aim was to purge the land of all vampires and humans. He intended to complete and use the Device - the Hylden race's ancient weapon of destruction.
The Device was buried beneath Nosgoth's capital: Meridian. The Hylden General (as Kain later called him) needed to gain influence over the city's humans before he could get it up and running. Eventually, his vampire army would have taken the capital. The Hylden General decided to create his own army, exploiting the humans' fear of Kain's forces. He rebuilt the long-defunct Sarafan order, and set himself at its head as the Sarafan Lord. Two hundred years after Kain refused the sacrifice, he met the Sarafan Lord in mortal combat outside of Meridian. The Lord used the Nexus Stone to protect himself from the Soul Reaver as they fought. Kain was defeated, and was widely presumed to be dead.
Victorious, the Sarafan Lord claimed the Soul Reaver for himself, and the Sarafan set about bringing Nosgoth under their control. He became a dictator, and over the next two centuries, they enforced his brutal regime while hunting and killing the remaining vampires. A few vampires became servants of the Sarafan Lord in order to save themselves. While all this was going on, the Glyph energy networks were being installed as a conduit for the Device.
At the start of Blood Omen 2, Kain awoke in Meridian. He wanted to take his revenge on the Sarafan Lord and, with the Cabal's help, he prepared for battle.
The first time their paths crossed, Kain was still far from ready. He was escaping with Umah from the Sarafan Keep when the Sarafan Lord stopped them. Kain was angered that the Lord wielded his sword, and his hunger for vengeance overcame his reason; he wanted to fight there and then. The Sarafan Lord did not see Kain as a real threat, and would have killed him and Umah had she not teleported them away.
Umah told Kain and Vorador about the Nexus Stone. Kain stole it from the Industrial Quarter, and learned about the Device in the process. Destroying the Device temporarily became his priority. He visited the Seer for assistance, but was forced to leave when the Sarafan Lord set fire to her home. Kain then went on to explore the Device and the Eternal Prison. He found two other vampires who had suffered on account of his archenemy; both more so than he. Magnus - Kain's former champion - had been robbed of his mind and sent to the Eternal Prison for trying to assassinate the Sarafan Lord. Janos Audron - the ancient enemy of the Hylden - had been imprisoned within the Device by the Lord, and used to feed the Mass.
Kain poisoned the Mass, and rendered the Device useless. He arrived at the Hylden City for his final battle with the Sarafan Lord, carrying the Nexus Stone. Their earlier roles were reversed as the Lord used the Soul Reaver against him. They fought above the Hylden Gate. Kain dropped the Nexus Stone into the gate, exposing himself to harm, but initiating its closure. With this act, no more Hylden could enter Nosgoth's Material Realm, and once the gate closed completely, all Hylden already in the land would die. Janos arrived to help Kain win, seeking a small measure of revenge for himself against the Lord. He was still weak, however, and the Sarafan Lord held him off easily. He threw Janos through the gate, sealing him in the demon dimension.
Janos' interruption allowed Kain to reclaim the Soul Reaver. With it, he finally killed the Dark Entity/Sarafan Lord. Nosgoth was Kain's for the taking.
Defiance bridged the gap in the storyline between Blood Omen and Blood Omen 2. It showed the Dark Entity's actions following his defeat by Kain at the Pillars, and explained how Janos had become imprisoned in the Device. Defiance dropped the name 'Hylden General', and instead referred to the Dark Entity as the 'Hylden Lord'.
In Defiance, Raziel met the Dark Entity/Hylden Lord in the Avernus catacombs, speaking through the possessed Mortanius. Raziel addressed him as the 'Unspoken', although in this game it seemed Ariel and Vorador had meant that term to refer to the Hylden race as a whole. Later, Raziel witnessed the collapse of the Pillars from the Vampire Citadel.
The Hylden Lord - having abandoned Mortanius' deformed corpse after the fight with the fledgling vampire Kain - possessed Janos Audron. The Hylden Lord gloated about his victory. He congratulated Raziel for providing him with a new host, the blood of their "ancient enemy" (Janos' blood, needed to feed the Mass), and for killing the Scion of Balance to do it. Raziel fought the Hylden Lord, but he couldn't bring himself to murder Janos, so the Lord ultimately triumphed. He destroyed Raziel's physical form, sending him back to the Elder God. The Hylden Lord flew away with Janos' body, thus making the events of Blood Omen 2 possible.
In Blood Omen 2, the Hylden Lord was inhabiting his own body, brought into Nosgoth's Material Realm from the demon dimension shortly after the Pillars' collapse. According to the second Defiance Q&A page at thelostworlds.net, this had been achieved by using Janos' body to open the Hylden Gate. As he was no longer needed as a vessel, Janos was imprisoned in the Device to feed the Mass, and there he remained for around four hundred years, until the younger Kain freed him.