Eternia Crystal

Eternia Crystals are purchased from the Tavernkeep for (pre-Mechanical Bosses),  (post-Mechanical Bosses) or  (post-Golem). The Discount Card and Greedy Ring have no effect on its price. Because of the price increase, you may wish to stock up before defeating the mechanical bosses. When used in the Eternia Crystal Stand, the Old One's Army event begins.

Due to the price increasing after Golem/mechanical bosses have been defeated, it is possible to buy Eternia crystals beforehand and sell them after the battles, netting a total profit of 55 silver per crystal.
 * This is not possible on the Switch version; they sell for even post-Golem when the full T3 DD2 item selection is unlocked..

The Event lasts for varying lengths depending on game progress:
 * Tier 1: 5 waves
 * Tier 2 (After defeating any Mechanical Boss): 7 waves
 * Tier 3 (Post Golem): 7 waves + Boss wave (Betsy)

The waves begin easy and get progressively more difficult until the final wave (wave 7 post and pre-golem) there is a boss as well as the invasion creatures. Defeating the boss finishes the wave, however.

History
Cristal Eternia 埃特尼亚水晶