Frigid Timewarped Prism

Frigid Timewarped Prism starts the quest The Unstable Prism.

Source
This item drops from the following bosses on Timewalking mode:
 * in The Nexus
 * in Utgarde Pinnacle
 * in Ahn'kahet: The Old Kingdom
 * in Gundrak
 * in Halls of Lightning
 * in Gundrak