The shape shifting ring is a ring that was given to Kimihiro Watanuki from the Jorougumo as payment for giving her The Crimson Pearl. The ring is a magical item that changes its shape when a person puts it on. The ring takes the shape of a person's spiritual energy. After being given to Watanuki, he passes it onto Shizuka Doumeki.
Shizuka first takes it and tries to put it on his right ring finger. An annoyed Watanuki instructs Shizuka to put it on his left index finger instead. Shizuka complains, as it did not fit his ring finger, but to his surprise, it fits perfectly.
For Shizuka, the manifestation was a bow. Shizuka was able to shoot from this bow with the thin glowing arrows that appear. The arrow could pierce through any spirit and exorcise them. It is unknown if the bow can be seen by a regular person or if it can only be seen by someone with magical sight.
When the object (the manifestation of spirit energy) is used, it transforms back into a normal ring. Watanuki warns Shizuka to keep it with him at all times for protection.
The shape shifting ring usually takes the form of a normal metal ring with something engraved on it. When Shizuka uses the ring, however, it has an arrow with smoke behind it engraved on the ring. Since the ring turns into a bow and arrow for Shizuka, it is unknown if the engraved symbol on the ring changes depending on the person who is holding it. Since the ring is not shown being held by anyone either than Shizuka or Watanuki, it is unknown if this is it's real appearance or not.