The Hand Demon was a notorious Demon that killed thirteen of Urokodaki Sakonji's apprentices, including Sabito and Makomo.

The Hand Demon is a large, veiny, hulking monster with olive green skin, red fingernails, and yellow eyes. He used to have a very normal demonic appearance, with regular hair and clothes, but after he started eating multiple humans, he started growing in size. He grew multiple arms and his mouth became huge, with jagged teeth. He grew so many arms that not only did he stop wearing his clothes, using his arms to cover himself, he also stopped using his legs, walking on a sort of hand stand.

(Source: Kimetsu no Yaiba Fandom)