Soul of Light

The  is one of the 6 souls that monsters can drop after the Wall of Flesh has been killed in the world. The Soul of Light can be dropped by ANY monster (excluding ones summoned by Statues) that die within the Underground Hallow, regardless of whether or not they spawned there. The video shown is wrong, he says that the only way to get them is to kill the " Pink Chaos Elementals ". 1: the only kind of Chaos Elemental is pink, and the real way to get souls of Light is to (as already stated earlier) kill anything in the underground hallow whether or not it spawned there.

You will need a total of 127 Souls of Light to craft all items, including a full stack of Light Discs.

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Potential Farming Methods
It is possible to make an artificial Hallow/Corruption Biome in the underworld that does not spawn Hallow Biome creatures. By placing at least 146 blocks of Ebonstone or Pearlstone Block so that they all are on screen at the same time, underworld monsters will begin dropping the respective souls. The players proximity to the placed stones is what causes the monsters to drop souls, if you move the placed blocks off-screen, the monsters will cease to drop souls, but monsters killed at the edge of the screen while the blocks are on screen will still drop souls.