Palpatine

Palpatine, also known as Darth Sidious, was the last Supreme Chancellor of the Galactic Republic and the first Emperor of the Galactic Empire before he was killed by his former apprentice, Darth Vader, at Endor. Despite his spirit inhabiting a prepared clone body at Byss and launching an offensive against the newly-established New Republic, Palpatine again met his end at Onderon.

He was again ressurected at the Valley of the Jedi by religious fanatic, Valnir, retaking his role as the Emperor of the newly-created Imperial Protectorate.

Last sighting
This information has been taken directly from the Star Wars Databank (located here)

The last the galaxy heard of the Emperor was nineteen years after the Battle of Endor, and more than a decade after the last of the clone bodies of Emperor Palpatine were destroyed, it seemed that he had arisen again and became head of a coalition seeking to reestablish the Empire. The Great Leader of the Second Imperium was supposedly the Emperor reborn yet again. The Emperor's image was transmitted regularly to the Shadow Academy. Although the academy leader, Brakiss, found it difficult to imagine how the Emperor had survived, he knew that with the Force, many things were possible.

The Great Leader was transported to the Shadow Academy sealed within a room-sized containment unit. Brakiss eventually discovered that the Great Leader was not Palpatine after all, but a series of recordings and props used by Four unnamed Imperial Royal Guards to trick the Galaxy into thinking that the Emperor had returned to rule the Second Imperium.

The God-Emperor
A secret society spawned from key members of the Moff Council, known as Palpatine's Children, set the seeds to return the Imperial Remnant back to it's former glory. One of the measures necessary was ressurecting the man responsible for it's start, Palpatine himself. This was done through harnessing the powers of the Valley of the Jedi and leaving the planet of Ruusan forever engulfed in a force storm.

While rumors circulated of his return, the Church of the New Order was created to spread faith and unite the galaxy under the belief that Palpatine was to be revered as a living God and a savior to the corruption and discontent plaguing the New Republic.

After the Battle of Coruscant, Palpatine declared the evolution of the Remnancy into the Imperial Protectorate with him as the Emperor once more.