This task can be unlocked by meeting with herbalist Tomira in her hut east from Sawmill (M1,16). It is best to start the quest while completing the main quest Beast of White Orchard. During the conversation you will learn about Lena who received serious injuries during griffin attack. You can offer making a mixture that will reduce the suffering of dying woman. This mixture is Swallow. Activate alchemy window and find the proper potion (it is shown on the picture above). To brew the mixture, you will need some ingredients - 1 dwarven spirit, 5 celandines and 1 drowner brain. First two ingredients can be found while exploring the game world or you can buy them from local merchants. Drowner brain can be obtained by defeating one of the water monsters and looting his body.
After collecting all the ingredients, brew the potion and return with it to Tomira. You will receive 350 experience points, 50 crowns, 2 venom extracts, manuscript page - Cursed Oil, torn-out page - Noonwraith Decoction and a book "Of Sweat and Blood". After reading the latter, you will learn about new monster types.