The Shadow Ring is a unique artifact, which can be crafted in the course of Torment: Tides of Numenera. You can do this by finding the two halves of the ring, examining them, and the joining them together to form the ring.
The first fragment of the ring is called Areph. There are two ways to acquire it:
1) When completing the side quest Anechoic Lazaret in Sagus Cliffs, you need to avoid confrontation with the drones and stealthily make your way to the terminals, where you can reprogram the drones. If you can do it, Salimeri will give you the Areph piece after you complete the whole quest.
2) Having reached the Caravanserai district in Sagus Cliffs, you may meet the Silver orphans, and inform them of any proof of intelligent behavior of machines you may have found. One of the rewards for such information will be Areph.
The second half of the ring is called Vareth and can be obtained once you reach the Miel Avest Sanctuary later in the game. The item can be bought from Lady Anshe.
Having acquired both pieces, open your inventory, choose any of them and select "use". Examine both fragments and combine them - you can do it by using either the Tidal Affinity skill or a Might check. If you pass it, you will receive The Shadow Ring, as shown on the picture. It will increase resistance of the character wearing it, and add a bonus to relativistic damage.