Reflection of Terror is an optional quest. Sin wants you to kill Yugul, Reflection of Terror, who can be found in the High Gardens. Completing this quest is a Path of Exile: Expedition challenge.
Reflection of Terror Guide
Start
Talk to Sin(location: The Sarn Encampment) after completed The Wings of Vastiri quest.
1 Passive Skill Points, Soul of Yugul(Pantheon power)
Kill Yugul
Location: The High Gardens(It is connected to The Bath House)
Walkthrough
Location: The High Gardens. Sin wants you to kill Yugul, Reflection of Terror, who can be found in the High Gardens.
Location: The Sarn Encampment. You have killed Yugul, Reflection of Terror. Talk to Hargan for a reward.
Quest Complete - You have killed Yugul, Reflection of Terror. Hargan has rewarded you with a Book of Skill. Use it to gain 1 Passive Skill Points. Also reward you Soul of Yugul.