Nexus Crystal

Nexus Crystal was an ingredient for high level enchantments and a blacksmithing plan.

Source
Nexus Crystals were disenchanted from high level epic and rare items at the following probabilities:
 * Item level 56-60 equipment: 1x
 * Item level 76-80 weapons: 1x (33%), 2x (67%)
 * All other Item level 61-80 equipment: 1x (50%), 2x (50%)
 * Item level 66-99 equipment: 1x (0.5%)

As an ingredient
Nexus Crystals were used in the following recipes:

Blacksmithing

 * Sageblade

Enchanting

 * Nexus Transformation

Before Patch 2.2.0

 * Enchant Cloak - Dodge
 * Enchant Cloak - Greater Fire Resistance
 * Enchant Cloak - Greater Nature Resistance
 * Stealth (enchant)
 * Enchant Gloves - Superior Agility
 * Threat (enchant)
 * Fire Power
 * Frost Power
 * Healing Power (gloves enchant)
 * Shadow Power