Cursed Flame

This is the Corruption half of the early Hardmode material pair, mirroring Ichor on the Crimson side. The enemies carrying it all live in the underground Corruption, so the practical approach is one properly built farm down there rather than wandering the chasms hoping for spawns.

You are not locked out. Buy Corrupt Seeds from the Dryad while a Graveyard is active in Hardmode, plant a pocket of underground Corruption somewhere isolated, and farm that instead. Hauling a stack over from another world works too and takes less setup, though the seeded patch keeps paying out. Either way, contain it: an artificial Corruption in a Crimson world spreads exactly like the real thing.

Most weapons made from it have an Ichor twin, so the effort is only worth it if you specifically want the green versions or the one item with no counterpart.

The idea of a flame that burns where fire should not is old. Greek fire, the Byzantine naval weapon that kept burning on water, is the likely root, with a nod to the green flames in Lovecraft's short story The Festival. Worth noting that Spazmatism spits the same stuff at you and drops none of it.