From Terraria Wiki
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- Blood Moons and New Moons are useful for collecting these, as Demon Eyes spawn more frequently. A common strategy is to make a tunnel underneath one block of any material and then cover those blocks in enough lava to kill the Demon Eyes, but not enough to burn the drops (a copper Coin can be used to test for the proper depth). A Water Candle and Battle Potion will further increase the spawn rate of Demon Eyes.