Eternia Crystals are purchased from the 酒馆老板 for 25 (pre-机械 Boss), 1 (post-机械 Boss) or 4 (post-石巨人). The 优惠卡 and 贪婪戒指 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 埃特尼亚水晶座, the 撒旦军队 事件 begins.
The Event lasts for varying lengths depending on game progress:
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.