im guessing thats the snowdin encounter, because they are sad in the snowdin encounter usually

Description: Trepidation - DEF: 666. DMG DEDUCT: 66.6%. ATK: 666. HP: 666. LV: 66 "be careful, you can only deal so little of your damage, be prepared... D U S T D U S T S A N S" Story and etc: https://www.quotev.com/story/15001179/Terror-Sans-Bio/1