I have checked around and there is mention that killing the EoW in the underground Corrupt Biome gives a lot of soul of night. I have tried this a couple of times, and not one single Soul of Night was dropped..

Did I do something wrong?

p.s :- I already killed the Wall of Flesh

  - No, you have not done anything wrong, the Eater Of Worlds cannot drop 'Soul Of Night' because it is possible to summon it before your world enters hard mode and before hard mode, you cannot get them. Currently, no bosses drop 'Soul Of Night' the best way I personally feel to acquire it is to go to anUnderground Corruption.