Dark Souls Remastered | How to get Dark Fog (Sorcery), starting from Oolacile Township bonfire

From the Dark Souls wiki:

"Dark Fog is a Sorcery in Dark Souls. This sorcery creates a cloud of poison around a targeted area, inflicting the Poison status. Discovered by an Oolacile sorcerer nearing madness, it shrouds the battlefield in toxic mist. To cast a sorcery, you must use a Catalyst or Special Weapons that can cast Sorceries."

You can read more about Dark Fog here:

https://darksouls.wiki.fextralife.com/Dark+Fog