The Stephen King universe is inhabited by a variety of characters, both human and supernatural, and of course, it has seen different villains – but could two of the evilest ones, Pennywise and Randall Flagg, be the same entity? Stephen King has terrorized generations of readers for years, and with time, he has referenced events and characters in different, unrelated books, making way for the Stephen King universe, which has built its own mythology, and with that, many mysteries and theories have emerged as well.

https://screenrant.com/stephen-king-pennywise-randall-flagg-same-entity-theory/