Sadako's evil spirit is based off of the Japanese concept of onryō (怨霊) or "vengeful ghosts." Onryō were thought to be the souls of those who died with extreme hatred, particularly women. They all have a specified appearance: pale women with long, disheveled black hair wearing white burial clothes. (In ancient Japan, women's hair was kept up until death.) Sadako herself is based off of two famous onryō: Okiku of Banchō Sarayashiki (番町皿屋敷, The Dish Mansion at Banchō) who was murdered and thrown down a well by the samurai she spurned, and Oiwa of the Yotsuya Kaidan (四谷怪談) who was fooled into drinking poison and murdered by her husband.

im a lil late but happy halloween