Heretics of Dune recap

Fifteen centuries after the God Emperor's fall, the Bene Gesserit sisterhood steers galactic affairs while a menace gathers. On the desert planet Rakis, a young girl named Sheeana is discovered who can command the giant sandworms, making her both holy and coveted. Meanwhile the Bene Gesserit protect their latest ghola of Duncan Idaho, grown for them by the Tleilaxu, and recall the aged master strategist Miles Teg to guard and train him. The great threat is the Honored Matres, a brutal matriarchy fleeing back from the far Scattering, who conquer worlds through overwhelming violence and a coercive sexual bonding that enslaves men. As the sisterhood maneuvers to understand and survive them, Teg is captured and tortured, an ordeal that awakens in him startling superhuman speed and perception. The Duncan ghola recovers the memories of all his past lives, and a confrontation with an Honored Matre named Murbella bonds the two of them to each other rather than enslaving him. The Honored Matres, enraged and unstoppable, attack Rakis and annihilate it, wiping out the sandworms. In the destruction, Teg sacrifices himself to cover the escape of his companions. Sheeana rides the last surviving worm to safety, and the Bene Gesserit flee with a captured worm and its sandtrout, carrying the means to grow new spice-bearing worms on their hidden homeworld, Chapterhouse, as they brace for open war with the Honored Matres.

Previously in the Dune series: the desert planet Arrakis, sole source of the spice that grants long life and makes space travel possible, shaped the fate of humanity for thousands of years. The noble Paul Atreides became the Fremen messiah Muad'Dib, seized the imperial throne, and unleashed a holy war before dying in the desert. His pre-born son, Leto II, saw that mankind would eventually destroy itself and set out to prevent it by taking the Golden Path. He fused his body with sandworm creatures and ruled as an immortal, worm-bodied God Emperor for some three and a half thousand years, holding humanity in a stifling, changeless peace. When he arranged his own death, he did so knowing the pent-up pressure of his tyranny would burst outward: after his fall, vast numbers of people fled into the unknown reaches of space in what became known as the Scattering. His death also destroyed most of the sandworms and threw the old empire into famine and collapse. Now, about fifteen centuries later, power is held by the Bene Gesserit, the ancient sisterhood of subtle, superhumanly disciplined women who breed bloodlines and manipulate politics from the shadows. The spice trade has revived, the worms have returned to the desert planet, now called Rakis, and the Bene Gesserit maintain their schemes with the help of the Bene Tleilax, who can grow living copies of the dead. But something dangerous is coming back out of the Scattering.

The book ends with catastrophe and a narrow escape. The Honored Matres, a savage matriarchy returning from the Scattering, prove far more powerful and ruthless than the Bene Gesserit anticipated. After the sisterhood's schemes on the desert planet Rakis come to a head, the Honored Matres attack the planet and destroy it utterly, obliterating the sacred sandworms and the worm-god religion built around them. The aged Bashar Miles Teg, transformed by torture into something faster and more perceptive than an ordinary man, uses his new abilities to organize the defense and buy time, and he gives his life in the fighting so that the others can get away. The young sandworm-commander Sheeana escapes by riding the last worm out of the doomed desert. The Duncan Idaho ghola, having regained the memories of all his former lives, flees together with Murbella, an Honored Matre whose attempt to sexually enslave him instead left the two of them mutually bonded. The Reverend Mother Darwi Odrade emerges as a decisive leader for the sisterhood. Carrying a captured sandworm and the sandtrout that can seed new spice deserts, the Bene Gesserit retreat to their concealed world of Chapterhouse, intending to grow their own worms and preserve the spice cycle. They are left facing open, desperate war against an enemy that outmatches them, with survival, not victory, as the immediate hope.