Hidalgo

In 1890, American Frank Hopkins (Viggo Mortensen) and his mustang, Hidalgo, are part of Buffalo Bill's Wild West show, where they are advertised as "the world's greatest distance horse and rider". Hopkins had been a famous distance rider, a cowboy, and a dispatch rider for the United States government; in the latter capacity he carried a message to the U.S. 7th Cavalry Regiment authorizing the Wounded Knee Massacre of Lakota Sioux.
Wealthy Sheikh Riyadh (Omar Sharif) has sent his attachÃ© Aziz (Adam Alexi-Malle) to ask the show to either stop using the phrase "the world's greatest distance horse and rider" or allow Hopkins and Hidalgo to prove themselves by entering the Middle Eastern "Ocean of Fire" race: an annual 3,000-mile survival race across the Najd desert region. The Sheikh is custodian of the al-Khamsa line, considered to be the greatest distance horses in the world, and traditionally the race has been restricted to pure-bred Arabian horses and Bedouin or Arab riders.
In addition to the grueling conditions, prevailing animosity and contempt for a Christian "infidel" and âimpureâ horse, horse and rider face stiff competition, including the wealthy and unscrupulous British horse breeder Lady Anne Davenport (Louise Lombard).
To complicate matters, Sheikh Riyadh has promised his daughter Jazira Jazeera (Zuleikha Robinson), his only surviving child, in marriage to the rider of his horse, should he win. She hopes to prevent this by giving Hopkins advice and information to help him win, thereby resulting in greater danger for them both. Sheikh Riyadh's outcast brigand nephew, who will stop at nothing to gain control of the al-Khamsa line, raids the race camp, kidnaps Jazira, and threatens to kill her unless he gets his uncle's prize stallion racer as her ransom. Hopkins manages to rescue Jazira. However, Davenport and the Sheikh's nephew try to sabotage the race by eliminating the rival riders, but are thwarted when Hopkins kills the nephew.
For Hopkins the Ocean of Fire becomes not only a matter of pride, honor and survival, but of identity as well: it emerges that his father was European American while his mother was Lakota Sioux. The Lakota call him âBlue Childâ or âFar Rider.â As a half-breed he feels sympathy and pity for his mother's people, but does not generally reveal his heritage, especially after the Wounded Knee massacre, for which he feels partly responsible. Jazira compares his relation to his heritage to her desire to avoid wearing the veil, saying that he mustn't "go through life hiding what God made you....like me."
Nearing the end of the race, Hidalgo is severely injured and Hopkins is dying of thirst. He considers shooting Hidalgo to end his suffering, but is unable to bring himself to do it. Kneeling, he chants a prayer to Wakan Tanka as a possible death song, and images of Lakota elders and his mother appear before him before Hidalgo suddenly struggles up, and Hopkins rides bareback to come from behind win the race. Hopkins wins the respect and admiration of the Arabs, and becomes friends with the Sheikh, giving him his revolver as a gift. As he bids farewell to an unveiled Jazira, she asks him if he is fulfilling the traditional Western tales' ending where the cowboy rides away into the setting sun and calls him Blue Child as she smiles kindly at him and turns to go.
Returning to the United States, Hopkins uses his winnings to buy a herd of mustangs about to be killed by government order, in what was an attempt by the US government to eliminate mustangs and force Native Americans to convert to farming. Hopkins has the horses released and frees Hidalgo to join them in the wild. The epilogue states that Hopkins went on to reportedly win 400 long-distance races and was an outspoken supporter for wild mustangs until his death in 1951, while Hidalgo's descendants live free in the wild in and around Oklahoma.

Question: Under whose custody is the al-Khamsa line?
Answer: Sheikh