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It looks like a green diamond (shape) and has a chance to drop from enemies from the Old Ones Army.

Upon putting the Eternia crystal in to the stand, you immediately get 10 Etherian mana.

10 Ethernian Mana crystals are consumed by using any Sentry Rod, Sentry Cane or Sentry Staff.

However the Eternian Mana crystals only occur in the event and vanish from the ground and from your inventory once the event is complete or has ended.