The Crystal Cave is a biome in Terraria: Otherworld that appears both on the surface and underground. It appears to be made out of a blue crystal, similar to the blue stone seen in the logo and multiple Purifying Towers.
The crystals from the cave seem to be the power source for most Purifying Towers:
- The player achieves [a pure world] by pushing back the Corruption until it is no more. Your primary method towards achieving this goal will be finding/crafting and then activating Purifying Towers. These machines of ancient design will push back anything unnatural in the world.
- Not content to simply allow the player to purify the land, the vile force behind the world’s corruption fights back. The resonance from the crystals in the purifying tower attracts enemies, driving them to destroy the towers and re-corrupt the player’s hard-won terrain.