"The Nexus Stone is an item of great power. It can bend time and space to create doorways to any location within Nosgoth." - Vorador
The Nexus Stone is an artifact that allows it's user to be protected from the physical attacks of the Soul Reaver. Strangely it looks like a bolotie but w/o the string, inscriptions on the metal like with the Sarafan (their armor) in Legacy of Kain: Soul Reaver 2. It can open portals to anywhere in Nosgoth (and presumably to anytime).