Planar Portals appeared in Soul Reaver and Soul Reaver 2. There was just one in Defiance.
After his execution in the Abyss, Raziel was a type of wraith: a creature of the Spectral Realm. He did not have a physical body, but with the aid of planar portals, he could gather matter and enter the Material Realm again.[1,2]
Luckily, these planar portals were plentiful in number, and easy to find throughout Soul Reaver and Soul Reaver 2, but Raziel could only use them when he was at full heath in the Spectral Realm. In Soul Reaver, the central 'fountain' of a portal would noticeably start to swirl around Raziel when he was at full health.
Standing on a portal, Raziel would cast the Shift Glyph. This would draw matter to him - giving him a real, solid, physical body - and the Spectral Realm would alter around him to become the Material Realm. Raziel did not need a portal to return to the Spectral Realm, he could abandon his physical form at any time.
It could have been inferred from Soul Reaver that the Elder God had created the planar portals, so Raziel could kill Kain and his mutated vampire brethren. However, in Defiance, it was conclusively revealed that the Elder God had some control - and possibly complete control - over the portals.
Raziel discovered a portal in the Cemetery, although it looked quite different to those in the other games. Before Raziel could use it, the Elder caused this and all other portals to vanish, saying: "Did you think to receive the same favors after your rebellion as before? No Raziel. I have no need for you to enter the physical world, so no conduit will be granted."
Raziel had to find another way to re-enter the Material Realm in Defiance. Unable to collect matter for himself from the environment, Raziel resorted to reusing old bodies: he possessed buried corpses and reshaped their dead cells and tissue to approximate his wraith appearance. This process did not require Raziel to be at full health.
- [DEF] The Elder God: "Did you think to receive the same favors after your rebellion as before? No Raziel. I have no need for you to enter the physical world, so no conduit will be granted. You serve me adequately as a wraith, and a wraith you will remain."
- [SR1] The Elder God to Raziel: "These portals are your conduit between the spectral and material realms. With their aid, you may gather matter and will yourself to become manifest in the physical world. This is taxing, however - your strength must first be fully restored. You require no conduit to return to this plane - you may abandon your physical body at any time."
- [DEF] Raziel: "In this crypt, I discovered ethereal gases rising from the mouldering corpse within. As I neared, I felt a distinct spiritual pull - and the closer I approached, the more that pull intensified. In the end, it was not difficult. I projected myself down into that tomb - and found myself reborn into the material world. It was a loathsome vessel - but with an effort of will, that too could change."