Dark Alchemy is a spell that yields a large amount of gold over a 20 second period. It gives you 750% of the amount gold you would get for killing the creature you are attacking for every auto-attack/TAP attack that you do during that 20 second period. Because you attack twice per second, that should be 40 attacks yielding gold, but in reality only 39 swings count.
The cooldown on Dark Alchemy, without Nanzuum Mana Cube, is 4 hours.
So if you're attacking a creature that gives you 1L gold when killed and you activate Dark Alchemy, you will get 7.5L per TAP attack for 20 seconds. 39 x 7.5L = 292.5L.
The Dark Alchemy spell is unlocked when you get the Cie Rune on your sword to level 5.
A powerful curse that transforms freshly spilled blood into precious metal.
Because you want to get all 39 attacks in during the 20 second window that Dark Alchemy is activated, it is imperative that you have Endurance activate prior to activating Dark Alchemy. This will ensure you get as many auto-attacks in as possible.
Dark Alchemy also works very well with Intensify, changing the gold output from 750% per attack to 1500% per attack and doubling the amount of gold you receive.