To obtain the Gloom-Borne Illness side quest in The Legend of Zelda: Tears of the Kingdom, speak with Lasli in Kakariko Village.
She’s looking for a porridge that counteracts gloom sickness. The ingredients you need to make this dish are Hylian rice, fresh milk and Sundelions. You can find Sundelions growing on the sky islands. If you need Hylian rice or fresh milk, you can purchase them from shops and wandering travelers.
Once you’ve crafted the Sunny Veggie Porridge, speak with Lasli to complete the quest. As a reward, the Armor Shop’s prices in down will drop.