Do the miniquest for Mark of Reliability.
Wait a few minutes after the sidhe spy dies to pick up the band of dull ivory from the ground.
Wear the ring and there is a random chance to release the sidhe spirit for the mark. The sidhe spirit may be trapped and sucked back into the ring, in which case the ring will not be removable for a short period of time. When it's removable again, you'll see the emote: "A band of dull ivory loosens its grip on your finger." Sometimes it may even "spark" without doing anything. Keep removing and re-equipping until it releases the spirit or traps it back.
<2937hp 2886sp 2957st> You wear a band of dull ivory on your right finger. A band of dull ivory glows white! <2937hp 2886sp 2957st> rec list A grumpy sidhe spirit says to you, 'Turn into an ice troll and wither in the fires of retribution!' A grumpy sidhe spirit begins casting a spell... A grumpy sidhe spirit has completed his casting! The temperature drops as a grumpy sidhe's spirit is released from a band of dull ivory! 'Aha!' says a grumpy sidhe spirit, 'Freedom! I will never forget you Dyna, my revenge will be sweet!' A grumpy sidhe spirit fades away. For releasing an imprisoned spirit, the Powers have granted thee a Mark. You have been granted the Mark of Confinement! You gain 181 quest points!