You need first kill 20 enemies before encountering Ruined Knights and then when you encounter them you need to spare knights and boom, Chara got shadow with visible eyes. Shadow with eyes also happens to be on other animations such as Chara’s falling in the sewer animation, equipping cowboy hat after Asgore’s hug, sitting on the bench and when Chara is sleeping in Asgore’s house. No new dialogue will be present as far as I know, it will be just neutral run.