The Chronoplast was the largest of the Time Streaming Devices in Nosgoth, and it once belonged to Moebius the Time Streamer. It is unclear whether or not he was its architect - he may have inherited it from his Time Guardian predecessor(s) - but it was capable of creating a portal that would allow him, or anyone else, to travel through time.
The Chronoplast device was built into a large room with multiple tiers. On each tier there were several clockwork mechanisms. By manipulating these mechanisms, Moebius could select the period of Nosgoth's history he wished to visit. A claw-shaped piece of machinery was mounted on the high ceiling (the ceiling looked like a dark, starry night sky) and it spun with increasing speed as the Chronoplast was activated.
The Chronoplast was concealed deep within a mountain to the north of Nosgoth. Kain visited the mountain in Blood Omen to see the Oracle of Nosgoth. He didn't realize that the Oracle was actually Moebius until much later, after the execution of Vorador. The meeting took place in the cauldron room of the Oracle's Cave, and satisfied with Moebius' advice, Kain did not progress beyond it. He was apparently unaware of the Chronoplast's existence for a while.
By the time of Soul Reaver, Kain was very familiar with the Chronoplast device. Sometime after Moebius' death at his hands, Kain probably returned to explore the cauldron room further, and found the hidden passage that led down to the Chronoplast. It is likely that his whole plan for shaping Nosgoth's destiny - including the execution of Raziel - was formed after observing Nosgoth's past and possible future through the Chronoplast, and the star portals in the corridor outside. As he said to Raziel in Soul Reaver, the Chronoplast offered "insight for those patient enough to look".
Raziel followed in Kain's footsteps in Soul Reaver. He discovered the secret passage from the cauldron room, and he negotiated the various obstacles between himself and the Chronoplast. He intended to confront and destroy Kain who was there, awaiting his arrival. When they met, Kain told Raziel how he had discovered the Chronoplast centuries before, and described the nature of destiny and fate: free will was an illusion.[2,3]
Raziel and Kain fought, and - in-between blows - Kain set the Chronoplast mechanisms, opening a new portal into the past. There is some discrepancy between Soul Reaver and Soul Reaver 2 at this point. In the former, Kain was visibly exhausted by the battle, which was more drawn out as the last 'boss' fight of the game. In the Soul Reaver 2 FMV (Full Motion Video) of the same events, he was supremely confident and easily rebuffed Raziel's attacks. At the end of the skirmish - in both games - Kain entered the Chronoplast portal, compelling Raziel to follow.
Raziel ended up in a Time Streaming Device within the Sarafan Stronghold, thirty years before the events of Blood Omen. Moebius had "snatched [Raziel] out of the time-stream" so they could meet. Kain was nowhere to be seen, and Raziel had to go to the Pillars of Nosgoth to find him.[4,5]
The Chronoplast could allow its users to travel through both time and space. This did not seem to be the case with the smaller Time Streaming Devices in Soul Reaver 2: when Raziel used them he always remained in the same device, he just found himself in a different era. It's a bit inconclusive however, as Raziel did not have an in-depth knowledge of how the devices worked. He couldn't adjust their target without Moebius' help, so we can't be sure what they were capable of.[7,6]
Kain and Raziel's dialogue in the Chronoplast varies a little between Soul Reaver and Soul Reaver 2. The Soul Reaver 2 version is shorter, and can perhaps be considered a recap. The only significant change is where "a millenium ago" becomes "aeons ago" (referring to when Moebius fortold Kain's future). This alteration makes sense; Raziel served Kain for a millenium before his execution, and centuries apparently passed before his return from the Abyss. Moebius' predictions couldn't have been that recent, as he was dead before Kain established his vampire empire.[2,8,9,10]
- [BO1] Kain (recognising Moebius after the execution of Vorador): "I had been betrayed. In my haste, I had not realized it before. That sigil on his forehead. The Oracle of Nosgoth was in fact the Time Streamer Moebius. And I had followed his advice!"
- [SR1] Kain to Raziel: "These chambers offer insight for those patient enough to look - in your haste to find me, perhaps you have not gazed deeply enough. Our futures are predestined - Moebius foretold mine a millennium ago. We each play out the parts fate has written for us. We are compelled ineluctably down pre-ordained paths. Free will is an illusion."
- [SR1] Kain to Raziel: "When I first stole into this chamber, centuries ago, I did not fathom the true power of knowledge."
- [SR2] Kain to Raziel: "Thirty years hence, I am presented with a dilemma - let's call it a two-sided coin.
If the coin falls one way, I sacrifice myself and thus restore the Pillars."
- [SR2] Moebius to Raziel: "Even now, Kain is lying in wait for you, unaware that I've snatched you out of the time-stream and brought you here to me. See how he lingers at the very Pillars he is destined to destroy, foolishly confident that he has eluded your grasp."
- [SR2] Raziel to Moebius (referring to the Time Streaming Device): "This era is of no further use to me. You will operate this device to provide me passage."
- [SR2] Raziel (referring to the abandoned Time Streaming Device in the Termogent Forest Swamp): "There was no way to tell what era the device was tuned to, and I had neither the knowledge, nor the means to set the machine myself."
- [SR2] Kain to Raziel: "Our futures are predestined -
Moebius foretold mine aeons ago. We each play out the parts fate has written for us. Free will is an illusion."
- [SR1] Raziel: "I stood with Kain and my brethren at the dawn of the empire. I have served Him a millennium."
- [SR1] Raziel: "In the instant between my execution and resurrection, centuries had apparently passed..."