Trained to perfection by Darth Plagueis, Darth Sidious was considered by many, including Darth Vader and Luke Skywalker, to be the most powerful Sith Lord in the entire history of the Sith Order. Despite his frail appearance, Darth Sidious was incredibly skilled in lightsaber combat, one of the greatest duelists of all time. A Sith Swordmaster, Sidious was one of the very few who was the equal of Count Dooku, Mace Windu and Yoda. In addition to his lightsaber combat skills, Darth Sidious was one of the most powerful Force users of all time, perceived in the Force by his apprentice, Darth Tyranus, as a "black hole of the dark side." Sidious' relationship with the dark side was so deep to the point that he became a possession of the dark, and the dark became a possession of his. As an apprentice under Darth Plagueis' tutelage, he learned all the known dark side powers of his predecessors, until he considered himself the Sith'ari. Sidious' mastery of the Force was such that even he at times wondered if the Force had been so strong in anyone before him, feeling a monster rising from the core of his self, impatient to unleash itself. Sidious was also the only known being capable of producing a Force storm of his own power, a dark side technique which he himself discovered, and that had enough power to kill an entire world. Using his anger and his will, he was able to rip the space-time continuum and create a hyperspace wormhole. Another very dangerous and advanced Force technique utilized by the Emperor was the ability to use the Force to transfer his essence which he used to survive death on at least two occasions. He used this power as a pathway to immortality. Among the Emperor's powers was the ability to channel the collective life essences of billions to sustain himself and his Dark Side Adepts; his victims would live in a dream-like state while his own power grew. The entire population of Byss lived under this spell until the planet's destruction by the Galaxy Gun. Upon spending decades immersing himself in the many facets of the Force, it was believed that Sidious had a mastery over all known powers, previously unknown powers, and could create new abilities on a whim. By the time of his final death, he had become a nexus of the dark side capable of tearing apart the fabric of space. His power was also such that even after death, it required every single deceased Jedi Knight to keep Sidious from escaping Chaos and wreaking havoc on the Galaxy.[