Why Is Paul Atreides's Son a Worm?

Paul Atreides never becomes a worm. His son Leto II does. In Children of Dune, Leto lets the larval form of the sandworm, sandtrout, cover his body as a living skin. It makes him nearly unkillable and lets him live for thousands of years, so he can rule long enough to walk the Golden Path. The price is that he slowly turns into a sandworm himself. (Children of Dune; God Emperor of Dune)

Paul stays human#

If you've heard that Paul becomes a worm, you're thinking of his son. Paul's own story ends very differently. Blinded in Dune Messiah, he walks into the desert. He comes back years later as a blind street prophet, the Preacher, and is killed on the steps of his sister's temple. The full story is on how Paul Atreides dies. (Dune Messiah; Children of Dune)

Paul's twins, Leto and Ghanima, are born at the end of Dune Messiah, and their mother Chani dies giving birth to them. Like their aunt Alia, they are born with the memories of their ancestors. (Dune Messiah; Children of Dune)

The Golden Path#

Nine years later, in Children of Dune, Leto fakes his death and goes into the deep desert. A spice overdose sends him into a trance in which he takes control of his ancestral memories and sees the future whole. (Children of Dune)

The future he chooses is the Golden Path. Humanity, left to itself, would grow stagnant and eventually be wiped out. The only way to prevent that is for someone to rule it with an iron grip for thousands of years, and then let it break free so widely that no single disaster could ever end it. (Children of Dune; God Emperor of Dune)

Paul had seen the same path and refused it because of what it would cost. When father and son meet in the desert, Leto tells him he is taking it on himself. (Children of Dune)

Why the sandtrout#

A plan that takes thousands of years needs a ruler who lives that long. Leto finds the means at the hidden sietch of Jacurutu. He lets sandtrout, the small larval stage of the sandworm, cover his body. They become a living skin over everything but his face. (Children of Dune)

The skin gives him huge strength and speed, makes him almost impossible to kill, and will keep him alive for thousands of years. He runs across the desert faster than any man. But it can't be undone. He will never again be wholly human, and the skin will slowly turn him into a worm. (Children of Dune)

So the plain answer is that Leto became a worm on purpose. It was the cost of living long enough to rule as the tyrant the Golden Path needed. (Children of Dune)

The God Emperor#

God Emperor of Dune jumps about 3,500 years ahead, to the last months of Leto's reign. By now he is a huge grey worm body with a human face, ruling from the last desert on Arrakis. His rule is a forced peace kept by an army of women, the Fish Speakers, and he controls the empire's whole supply of spice. (God Emperor of Dune)

He dies when the rebel Siona and a Duncan Idaho ghola drop him from a bridge into a river. Water is poison to sandtrout. His living skin tears loose and swims away as thousands of sandtrout. (God Emperor of Dune)

Those sandtrout will turn Arrakis back into desert and bring the worms back, and each new worm carries a pearl of Leto's awareness in an endless dream. In Heretics of Dune, set about 1,500 years later, the worms have returned. (God Emperor of Dune; Heretics of Dune)

Why it matters for Paul#

Our reading, and it's an interpretation, is that Leto's worm body is Herbert's answer to Paul. Paul saw the price of saving humanity and walked away, keeping his humanity and dying powerless. Leto saw the same price and paid it, giving up his human body to become the worst tyrant in history on purpose. (Children of Dune; God Emperor of Dune)

For more on Paul's children, see Paul Atreides's children. (Dune Messiah; Children of Dune)