The Emperor's Blade was a knife that originally started out as an heirloom of House Corrino, and ended up being enshrined in a temple dedicated to Paul Atreides. The blade was a long dagger, with a hilt that was jewel encrusted.
Shaddam Corrino IV & Duke Leto
The blade was a family artifact that was handed down from Emperor to Emperor, during the time of the Corrino Empire. When Crown Prince Shaddam Corrino IV testified on behalf of the young Duke Leto Atreides I, at Leto's Trial by Forfeiture, Shaddam ended his fervent defense of Leto by giving him the elongated dagger as a sign of their deep friendship.
Nineteen years later, after Shaddam's great humiliation at the end of the Great Spice War, the Padishah Emperor faced possible censorship of the royal Crown at a special convocation of the Landsraad. After Shaddam's half-hearted, and near hypocritical defense of his actions of the previous two years; Duke Leto stepped in to defend his cousin, gave a spell-binding speech in defense of House Corrino. As a symbol and token of fealty to the Emperor, Leto gave the blade back to Shaddam.
As the Ascension of House Atreides occurred on Arrakis, and the final duels took place just before Paul took the Imperial Chair, Shaddam gave the Na-Baron Feyd-Rautha Harkonnen the blade to fight his life-or-death duel with the Atreides heir. When Paul killed Harkonnen, Shaddam attempted to give the blade to Count Hasimir Fenring, and encouraged his friend to fight young Atreides, which Fenring refused. Shaddam then took the blade with him into exile to Salusa Secundus.
Count Hasimir Fenring
In 10193 AG, a few years later, the deposed Emperor sent the dagger with his Sardaukar leader, Bashar Zum Garon to Tleilax, as a gift to Fenring. Shaddam hoped to entice Hasimir to join him on Salusus Secundus in his vain attempts to overthrow Emperor Paul Atreides, which Fenring refused. Fenring stayed on in Thalidei on the Tleilaxu planet a year longer, with his own secret assasination attempt coming together in his and Margot Fenring's minds.
When Hasimir, Margot, and their daughter Marie Fenring; joined the Imperial family for a celebratory banquet on Arrakis a few months later, Hasimir's trap sprang into action. Marie used a needle-whip which had been attached to the banquet table leg, as a tool to stab Paul with, but Alia Atreides, Paul's sister, stabbed her deeply first, and killed her.
As both Fenring's mourned the loss of their daughter, Margot shouted out to the Emperor that Marie was Feyd's daughter, which caused Paul to drop his guard for just a moment. Hasimir then stabbed Paul mortally with the Blade again, which almost killed Muad'Dib; but using his Bene Gesserit powers, he was able to self-heal himself. The blade was taken by Korba, and placed in 'religious' safe keeping, where eventually in ended up in a Caladan temple/shrine dedicated to Paul.
Five thousand years later, the Enhanced Face Dancer Khrone found out information about the Emperor's Blade, and took his servant, the Lost Tleilaxu Scribe Uxtal to Dan examine the artifact. It had passed into the hands of the Fish Speakers, and was being guarded by a Cult of Sheeana priestess named Ardeth. Khrone and Uxtal found that the blade still possessed cells of Paul, and they killed Ardeth and took the Blade back to Bandalong on Tleilax. They placed the cellular material in one fo their Axlotl Tanks, and created a clone of Atreides, whom they named Paolo.
Eleven years later on the Day of Kralizec, after Paolo had reached the age of maturity, the Thinking Machine ruler Omnius and Erasmus captured the no-ship, the Ithaca. Feeling that they had completed their task of finding the Ultimate Kwisatz Haderach, both young Paolo and the ghola of Paul Atreides from the Ithaca, were brought to the capital of the machine-planet of Synchrony, at the Cathedral.
There, Paolo was presented with the Blade by Erasmus, in his duel-to-the-death with Paul in front of the Ithaca crew leaders; and the leaders of the Machine Empire. As the Battle concluded, Paul lost his Crysknife, to a magma fountain that bubbled in the Cathedral. Paolo then stabbed Paul deeply with the Blade, which caused the young Atreides ghola to journey to the verge of death, but Paul was once again, able to self-heal from the deep wound.
What became of the Blade afterwards is not known, when the ghola of Duncan Idaho was revealed to be the Ultimate super-being.
Behind the Scenes
The Emperor's Blade was not called such in any of the Original or Expanded Dune novels. Such an import has come to be placed on the Blade as it has existed in both Frank Herbert and Brian Herbert/Kevin J Anderson's novels, that fans have come to call the dagger, the Emperor's Blade.