The Hylden Gate appeared in Blood Omen 2. It had been constructed by the Sarafan Lord, and it was designed to allow him entry into Nosgoth from the Hylden's place of banishment: the demon dimension. When Kain refused to restore the Pillar of Balance in Blood Omen, he "caused a rift throughout the world, sufficient to breach through the dimensions". The Sarafan Lord used the gate, and he established (or, more likely, re-inhabited) the Hylden City. Over time, he brought others of his race through the gate to populate the city, although their numbers remained fairly limited.
In Blood Omen 2, Kain learned that the gate sustained the Hylden's existence in Nosgoth. It was not merely a door; it was "their umbilical cord to the other world", and they could not survive in Nosgoth without it. Kain went to the Hylden City with the intention of destroying the Hylden Gate (and the Sarafan Lord) for good.
Kain found and fought the Sarafan Lord at the Hylden Gate. The gate itself was a swirling greenish vortex, and their encounter took place on a structure of metal walkways above it. Kain knew he had to throw the Nexus Stone into the gate in order to close it, but he also wanted the stone to defend himself against the Soul Reaver, held by the Lord. Faced with a stalemate, Kain chose to discard the stone, knowing that it would cause the Hylden's demise, no matter what happened to him. The Sarafan Lord's Hylden army would be unable to invade Nosgoth.
The battle continued as the gate's walkways started to collapse around them. Kain eventually got the Sarafan Lord to drop the Soul Reaver, and Janos Audron interfered to help Kain claim it. His intervention proved costly, however: the Sarafan Lord threw Janos through the failing Hylden Gate, trapping him in the demon dimension with uncertain hope of rescue or escape. The final stage of Kain's fight with the Sarafan Lord took place on the tiny central part of the gate structure, and once the Lord was dead, Kain left the Hylden Gate in ruins. He considered the future of Nosgoth as it exploded behind him.
Since Defiance was released, a few more details about the Hylden Gate's activation have emerged. It was revealed that the Sarafan Lord (also known as the Hylden Lord) was the Dark Entity of Blood Omen. He had been active in Nosgoth before Kain's refusal at the Pillars, loosely possessing Mortanius for at least thirty years prior to that event. Apparently he had not tried to open the gate during that time, suggesting he couldn't, or perhaps just didn't want to until the time was right.
It seems he needed to wait for the "breach through the dimensions" before the gate could be used. Also by that time, Raziel had helpfully resurrected Janos Audron, providing the Hylden Lord with a new "incorruptible vessel" to possess, and the Ancient Vampire blood he required for the Device. It has been confirmed by Crystal Dynamics that the Sarafan Lord utilized Janos' controlled body to open the gate, bringing his own, Hylden body into Nosgoth.¹