Path of Exile: Delirium turns up the difficulty, adds more skill tree
Companies, agencies, institutions, etc
Mirror
Ambush Strongboxes
Incursion
the Mirror of Delirium
Cluster Jewels
the Delirium League
Jewel Clusters
Grinding Gear Games
Kinetic Bolt
Metamorph League
People
Blade Blast
Groups
Unique
Physical locations
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Places
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Locations
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Events
Grinding Gear Games
Summary
Players can earn these Cluster Jewels during the Delirium League, and attach them to a jewel socket at the edge of Path of Exile’s Passive Skill Tree.
As said here by Ryan Gilliam