Which Stephen King series features the Crimson King as the main antagonist?

The Stephen King series featuring the Crimson King as its main antagonist is The Dark Tower series.

The Crimson King is a trans-dimensional embodiment of evil who seeks to destroy the Dark Tower, the structure that holds the multiple worlds of King's fictional universe together. His ultimate goal is to rule the chaos that would remain if the Tower fell.

Although the character appears in other books, including Insomnia and Black House, The Dark Tower presents his largest role and establishes him as the ultimate enemy behind many threats facing Roland Deschain and his ka-tet. He also commands or influences villains such as Randall Flagg.

The Crimson King is not the same character as Pennywise from It or Randall Flagg from The Stand, though all are connected within King's wider multiverse. Flagg serves the Crimson King in The Dark Tower, while Insomnia introduces the Crimson King earlier in publication order. In the final Dark Tower novel, the King is trapped on a balcony of the Tower and ultimately erased through Patrick Danville's supernatural artistic power.