Tertius Eileen Anteac was a Bene Gesserit Mentat Reverend Mother and Truthsayer during the last years of the reign of the God Emperor Leto Atreides II. According to the God Emperor's knowledge, Anteac was a descendant of the famous Mother Gaius Helen Mohiam. She had a scar in her forehead, and served faithfully the plans of the God Emperor concerning the Ixians.
She went to Arrakis in 13725 AG together with the Reverend Mother and also Truthsayer Marcus Claire Luyseyal for an audience with the God Emperor which coincided with the Royal Festival held every ten years. She and Luyseyal received the message the assistant to the Ixian Ambassador, Othwi Yake, that Face Dancers had infiltrated in the Ixian Embassy and were planning to assassinate Leto II. The message did not reached Leto's hands in time, but anyway it failed, as both Anteac and Luyseyal knew it would. They achieved little in their meeting with the God Emperor, and he took the priceless spice with which they expected to test his mortality.
Later, Leto enlisted Anteac's aid in detecting the Tleilaxu Face Dancers who had replaced almost everyone in the Ixian embassy, except the new ambassador, Hwi Noree. During her mission, Anteac identified the duplicate of Othwi Yake, since the original had sent her the message of the Assassination Attempt against the God Emperor. Then, Moneo Atreides, Leto's Majordomo, recognized that Anteac was a secret Mentat, a skill prohibited in the Empire, but Leto afirmed that he knew that already and that that amused him.
Hwi Noree, the new Ixian Ambassador, shared her knowledge of the environment in which she was brought up with Anteac, who had been conscripted by Leto to lead a Fish Speaker assault on Ix, in order to wrest the secret of Hwi's origins. When the notice of Leto and Hwi's marriage was going to take place, Anteac was shocked, and at the same time, annoyed that her order had allowed such a talented woman as Hwi to pass through their training without turning her into one of them, a true Bene Gesserit sister.
Anteac lead the Fish Speaker invasion of Ix, where they captured Malky, the former Ixian Ambassador. However, the faithful Anteac died during the mission.