The Orb of the Emissary was the tenth orb sent out of the Bajoran wormhole by the Prophets. Unlike the other orbs, it could only be opened by the Emissary of the Prophets, Benjamin Sisko, who did so in 2375. The orb held a Prophet who was able to fend off an attack from the Kosst Amojan.
In the early 2330s, a Prophet had possessed Sarah Sisko for the purpose of ensuring Benjamin Sisko's birth to take on the role of Emissary. She owned a necklace which contained the phrase "Orb of the Emissary" in ancient Bajoran. Her husband, Joseph, would keep this for many years and eventually give it to his son, unknowingly helping him to discover the orb. The Prophet left Sarah in 2333 and stayed inside the orb, located buried in a desert on Tyree, its existence unknown for years.
In late 2374, Dukat allowed himself to be possessed by a Pah-wraith, which he later brought to Deep Space 9. There, the Pah-wraith fatally wounded Jadzia Dax, left Dukat's body and entered an Orb to attack. At the same time, all of the other eight Orbs went dark and the wormhole's entrance collapsed, leaving Benjamin Sisko and the Bajoran people unable to contact the Prophets. (DS9: "Tears of the Prophets")
Three months later, in 2375, Sisko received a series of visions that hinted at the existence of another orb and its location, while also leading him to uncover that the woman he had known as his mother was not the one who gave birth to him. Once he discovered the necklace, and therefore her connection to the orb, he became determined to find it, despite being attacked by a member of a Bajoran cult that believed that the Pah-wraiths were the true Prophets. (DS9: "Image in the Sand")
Sisko traveled to Tyree to find the orb, however, while attempting to retrieve it, he received a vision intended to dissuade him from retrieving the orb and using it. He began hearing voices calling a Doctor to help with a patient, culminating as he unearthed the Orb itself in a vision reminiscent of his previous visions involving himself in the role of Benny Russell, only now he was locked away in an asylum and scrawling his stories on the walls of his room. Despite this, he successfully accomplished his task. When the box containing the Orb was opened, the Prophet that had previously inhabited Sarah Sisko's body was released from it. The Prophet thanked the Emissary for fulfilling his task and releasing her, then returned to the wormhole, casting the Pah-wraith out, and restoring the wormhole and the other orbs. (DS9: "Shadows and Symbols")