Rift of Radiance

Rift of Radiance is a rift that brings you to the Otherworld. You can enter the rift after finding and activating the Altar of Radiance.

It's possible there are other results but from my experience, I've only gotten the Eternal Prison quest after entering.

In the Eternal Prison, you fight Apollonas the boss (easy fight)

It gives the same drop as a standard boss, so some stardust and possibly a unique memory or essence.