After the Emperor's Herald leaves Caladan in Dune, the Duke and ducal heir, Paul, discuss one-on-one the challenges that the departure to Arrakis brings. Duke Leto Atreides remains cautious of his mortal enemies, the Harkonnens. This rival house has amassed immense wealth from controlling the spice trade and holding Arrakis in a quasi-fief under a CHOAM Company contract for eighty years. He is aware that the Emperor is his hidden enemy, and tells Paul that the man has practically stirred a war between them and House Harkonnen by granting them the stewardship of Arrakis.
Duke Leto seeks to tread carefully on the desert planet and wants to appease its natives, the Fremen. If the two were to ally, they could achieve mutual goals. Leto says that the Atreides-Fremen alliance benefits the latter and puts an end to the prolonged cruel treatment they met at the hands of the Harkonnens. To that end, Duke Leto sends one of his most capable men, Duncan Idaho, on a scout mission to Arrakis a few weeks before the rest of them.
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Duncan Idaho Leaves For Arrakis
Frank Herbert's Dune describes the Swordmaster of Ginaz, Duncan Idaho, as Paul's companion-teacher on Caladan. Duncan leaves for Arrakis at the beginning of Villeneuve's film adaptation of Dune. While Paul desperately wants to accompany him, Leto refuses, saying he needs him by his side. Paul and Duncan exchange a few words before the latter departs for Arrakis with the advanced team. Paul takes an upfront position about his Arrakis dreams. Duncan is visibly shaken as Paul tells him:
I saw you lying dead. Falling in battle. It felt like if I had been there, you'd be alive.
Paul's words are interspersed with visuals of dead soldiers lying across a stone floor, and Duncan happens to be one of them. Duncan remains adamant that he's going to survive whatever awaits the Atreides cohort and says:
Dreams make good stories. But everything important happens when we're awake. Because that's when we make things happen.
With that, Duncan bids Paul goodbye with the hope that they will meet again.
Duncan Idaho's Teachings
Duncan's scout mission is a success, and he learns quite a deal during his time with the Fremen. His counterpart, Gurney Halleck (Josh Brolin in Dune) is happy to have him back, as is Paul. He made friends with the Fremen and ordered the security to take care of them on Arrakis. He wears a Fremen Stillsuit and carries a long sword. With his success, Duncan has become the first of Atreides' cohort to venture into the deserts of Arrakis:
For four weeks, I lived with the Fremen. Hidden in the desert in a community called a Sietch. Stilgar, the leader of that Sietch, has come to meet with you, sire.
The Sietches, as Duncan explains, are underground. He adds that Arrakis is full of them. Met with this knowledge, Duke Leto is visibly excited to utilize desert power to bolster the position of his House and become the Landsraad's greatest. Duncan adds that the Fremen fight like "demons," and acts as a bridge for their leader, Stilgar, whom he has brought with him to the Arrakeen Residency.
Duncan Idaho's Fate
Duncan's stey in the Sietches of the desert and his interactions with its people deeply affects Paul. Thanks to him, Paul understands that it is not for outsiders like them to keep the natives in their place. He learns that outsiders come solely for the Spice and give nothing in return to the Fremen. Paul is in awe as the proud Stilgar lays down his terms before the Duke.
Later, at dusk, Duncan presents a Paracompass to Paul at the military barracks, explaining the ingenuity of its technology. In admiration for the Fremen and the desert, Duncan talks about the "ingenious" things the natives have constructed. Stillsuits and Sand Compactors are a few of them. He describes the Fremen to Paul:
They're fierce, but loyal. Attuned to the desert. Part of it. It's a part of them. Wait till you see. It's beautiful out there.
Duncan escapes from Arrakeen Residency during the night attack and reunites with Paul and Lady Jessica in the latter half of Villeneuve's Dune. But before that, he has a face-off in the desert with the Imperial Planetologist/ecologist and the Judge of the Change, Liet Kynes.
Duncan lays down his life fighting the Sardaukar soldiers in an abandoned ecological testing station. He is dead as of Villeneuve's Dune: Part One, and it is highly unlikely that he will return in the movie sequel as it covers the second half of Herbert's first book. That said, Herbert's second novel, Dune: Messiah brings Duncan Idaho back as a Tleilaxu Ghola — reconstructed from the dead flesh of the original. The character's return is possible, provided Villeneuve's Dune 3 is made. Here's hoping Jason Momoa gets to embody the assassin named Hayt in the future of Dune.
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