Miles Teg was a highly regarded Bene Gesserit military commander who held the rank of Bashar. He was a descendant of Duke Leto Atreides, and a Mentat.
Background
Teg, who lived some 1500 years after the death of Leto II, was a product of the Bene Gesserit breeding program, down the line of Atreides genes. He was the eldest son of Loschy Teg and Janet Roxbrough, and from an early age was conditioned to serve the interests of the Bene Gesserit.
He was regarded as an excellent example of pure breeding (whereby he possessed all of the physical and mental attributes of a classic Atreides male), to the point where he was said to resemble Leto I almost exactly, with the exception of being somewhat taller. Indeed, when Teg first saw the ego-likeness of the Duke Leto in a Bene Gesserit library, he had regarded the experience as looking into a mirror; suggesting that not only did he possess the facial features as the old Duke, but his mannerisms as well.
Teg's training through the Bene Gesserit involved much of the learning instilled in Reverend Mothers. This was largely attributed to Teg's mother, who was a Bene Gesserit Reverend Mother herself. Therefore Teg was both a Mentat and a Bene Gesserit adept, a combination that made him a formidable military commander for the Sisterhood.
Reputation and Abilities
Teg had spent his career commanding Bene Gesserit military forces. He had developed a reputation for superior ability in successfully commanding men in battle, as well as engendering truces between implacable enemies. His successes were attributed to his genetic inheritance, since the men of the Atreides line were known to be charismatic and could elicit fierce loyalty in those who followed them. This potential was capitalised upon by the Bene Gesserit Sisterhood through their training of him.
Training the Ghola
Teg retired from a successful career to his home planet of Lernaeus. However, some time after his retirement, he was called back to active duty to train a Duncan Idaho ghola for the Bene Gesserit, as well as to activate his dormant memories and personality. The Sisterhood believed that Teg's superior abilities, as well as his resemblance to Leto I would assist in this matter, since the original Duncan Idaho had maintained a strong relationship with the Atreides Duke.
After Teg had managed to restore Duncan's memories, Tleilaxu forces attempted to capture him and Duncan, using both Face Dancers and people returning from The Scattering. However, Teg used several cunning, unexpected, and brilliant military tactics to help Duncan and Lucilla escape the attack, but Teg himself was captured. During his subsequent interrogation, Teg was tortured severely with a T-Probe. The extreme pain he experienced during this torture awoke prescient abilities in him, as well as extremely heightened physical and mental abilities that allowed him to overpower his interrogators and escape.
Death and Rebirth as a Ghola
Following his escape Teg gathered many retired war veterans from Gammu who had served under him. He then travelled to Rakis where he created a diversion to allow Duncan and Reverend Mother Lucilla to escape. Teg was killed in this diversion, which was a massive battle involving millions between forces of the Bene Gesserit and the Honored Matres.
However, some years later he was brought back from the dead as a ghola by the Bene Gesserit. In a curious reversal of roles, the original Miles Teg memories were awakened in the ghola by Sheeana (under the direction of Duncan Idaho). Duncan theorized that Sheeana could reawaken Teg's memories via Imprinting, but an original memory of hypnotrance suggestion (by Teg's mother) to repel Imprinting caused the necessary trauma for Teg's original memories to be restored. He then commanded forces which ultimately led to the subjugation of the Honored Matres under the Bene Gesserit.
After the merging of the Honored Matres and the Bene Gesserit, Ghola Teg left to join the scattering led by Sheeana.
Significance
Teg was a successful example of genetic breeding for the Bene Gesserit. He possessed all the powerful traits of the Atreides of the past, but unlike Paul Atreides and his son Leto II, Teg remained under complete control of the Sisterhood. This was because Teg's mother had remained loyal to the Sisterhood when the Lady Jessica had not. As a result, Teg's mother had managed to instill awe and loyalty for the Sisterhood from an early age through his upbringing and training.