Soul of Night

Depth is the condition that catches people. The world has to be in hardmode, you have to be inside Corruption or Crimson, and the kill has to happen at cavern depth or below, which includes the Underworld. Standing in a patch of surface evil biome produces nothing at all. Once those hold, almost anything that dies counts, including enemies with no connection to the biome. Remix and Zenith worlds drop the layer requirement entirely, so kills at any depth pay out there.

• Statue spawned enemies and boss minions, because neither leaves a coin behind.
• Anything with a single point of health, which rules out spells, spores and similar filler.
• Bosses themselves, Meteor Heads, winged Slimers, and a short list of slimes.

Dig a flat chamber deep inside the evil biome and wall off the edges so spawns funnel toward you. Run a Water Candle and a Battle Potion. Weapons that clear a whole room matter far more here than single target damage. The souls do not fall when they drop, they hang in the air where the enemy died, which makes them easy to spot and just as easy to leave behind if you are moving fast.

Working through everything these build runs well past a hundred souls, and the boss summoning items and the Key of Night sit on top of that total. Over farm in a single sitting. Digging down and setting up the arena costs more of your time than the killing does, so three separate trips pays that cost three times. Cool Whip, the Golden Shower, the Onyx Blaster and Spirit Flame are the usual reasons to be down there.

Expert worlds used to roll a second independent chance at a soul on every qualifying kill, which made them noticeably faster to farm. That was removed. The rate is identical on every difficulty now, so there is no argument for grinding these on a harder world than you actually want to play.