Darth Sidious, also known as Palpatine or The Emperor was the supreme ruler of the most powerful tyrannical regime the galaxy had ever witnessed, yet his roots seem extremely humble, traced back to the peaceful world of Naboo.
No one is quite sure how Palpatine was first introduced to the power of the dark side. He was the most powerful practitioner of the Sith ways in modern times. He studied the ancient ruins on the Sith mausoleum world of Korriban. He unlocked secrets of the Force from a captured Jedi Holocron. The dark side energies flowing through Palpatine's body were so intense, that they ravaged his mortal frame.
History
In 3 BBY, Galen Marek's training was nearly complete, and Darth Vader sent him to various missions to prove himself ready: killing rogue Jedi and dueling the simulated Sith spirits of Darth Desolous and Darth Phobos. In order to keep his presence hidden from the Emperor, Galen had to eliminate all witnesses--Imperials as well as resistance fighters. His final mission was to find Jedi Master Shaak Ti, the last surviving member of the Jedi High Council other than his former master Obi-Wan Kenobi, Yoda and Lord Vader himself, and kill her. Marek was successful, but, upon his return to the Executor, he and Vader were surrounded by the Emperor's fleet. Vader stabbed his secret apprentice and, under Palpatine's orders (through Starkiller's holodroid, PROXY), used the Force to throw Galen out of the viewport.[59] This, however, was a ruse, as a droid picked up Galen's body and took him to recover.
6 months later, the resurrected Galen was sent by Vader to assemble an army of the Emperor's enemies to oppose the Sith Master. Unfortunately for him, Vader captured the Rebels and revealed to Galen that it was all a ruse to have them reveal their treachery. Galen soon proved resistant after escaping and defected to both the Jedi and the Rebellion, this time of his own volition. He confronted the Sith Lords aboard the nearly completed Death Star, and stunned Palpatine by defeating Vader in combat.
Darth Sidious, who generally used his allies until they outlived their worth, was actually pleased that Galen was the victor of the duel. Ever since Mustafar, the Emperor had viewed Vader as a broken shadow of his former self: Anakin Skywalker. For years, Sidious was bitter about the fact that he had spent a lifetime trying to obtain an apprentice of Skywalker's caliber, and just as he succeeded, his prized pupil was forced to become "more machine than man," thus dashing any hopes the Emperor had of making Vader a powerful Sith Lord.
In Galen, Sidious finally saw an opportunity to rid himself of his hobbled disciple. Galen, by defeating his own master in combat, proved that he had the potential to be the perfect apprentice that Sidious had been searching for. Sensing what the Dark Lord had in mind, Kota telekinetically snatched Palpatine's lightsaber and attempted to cut him down with it, but the Emperor unleashed a barrage of Force lightning upon the charging Jedi.
Using the Force, Galen hurled bits of broken transparisteel and debris at the Emperor, who was forced to break off his attack. Galen's ranted as he came to the conclusion that Palpatine was behind Vader's every move ever since his discovery on Kashyyyk. Palpatine confirmed this, telling Marek that Vader never plotted alone.[60] Cackling, Palpatine abruptly dropped to his knees and goaded Marek into striking a death blow. With Kota's urging, however, Galen resisted the seduction back to the Sith way and spared the Emperor's life. As Galen turned to leave, Palpatine leapt to his feet and again attacked Kota with Sith lightning for speaking out. Galen, in response, dove in front of his friend and deflected what he could. With his dying breath, Galen caused an explosion with the Force, staggering Palpatine long enough for a group of captured Rebel leaders to escape.
Palpatine took no joy in the death of Galen Marek. He had envisioned a far different future for the boy, as he once had with Anakin. Galen was meant to root out the rebels so that they could all be revealed and destroyed, yet Sidious bitterly admitted that Galen's sacrifice would only inspire them to fight a full-scale war against the Galactic Empire. On a more personal note, the Emperor had lost what was supposed to be Vader's successor as his new apprentice. To have lost someone of Galen's potential and having to still make do with a cyborg apprentice was a frustrating setback. All in all, the combination of Galen's death and the survival of the Rebel leaders led Darth Sidious to the wise conclusion that unless he destroyed every single rebel now, the Rebellion which he and Vader unwittingly helped to create, would be the undoing of the Empire and even the Order of the Sith Lords itself.
The Emperor, left with no alternative now that Galen was dead, was forced to keep Darth Vader as his slave until he could find a new apprentice, and then, like Darth Tyranus, Vader would be swept aside. However, the incident with Galen also served to drive the master and his apprentice even further apart then they already were. Like Tyranus before him, Vader had been betrayed by his master in favor of an apprentice far younger and more powerful. And it would not be the last time that the Emperor would try to replace the half-mechanical Sith Lord with a worthier apprentice, as the encounter between the Sith Lords and Luke Skywalker would later reveal.