This essence is used to summon a harbinger monster that drops conjurer's loot and is the yellow colored version. Crystallized Essence from Ultima Online was only available in 2009 during the Halloween event.

Red - Was to be given to Thomas the Toymaker for Halloween Costumes. After the first, 5 more were required per costume.
Blue - Was used to heal gypsies who have used Jared's potion. When successfully cured, they would give a Conjurer's Garb that included 140 luck.
Yellow - Summons a Harbinger monster which is as tough as as champion or peerless. This mobster drops special loot and can still be summoned today regardless of the Halloween event.