Darth Sidious

Throughout most of the galaxy, Sheev Palpatine, also known as chancellor Palpatine and Emperor Palpatine, Is the leader of the Republic/empire. Loved by the old Republic and dreaded by the Rebel alliance, Palpatine was known as a effective but cruel leader.

But to few He is feared as Dark lord of the Sith Darth Sidious, a powerful dark sided and came from a long line of Sith from Darth Bane. Darth Sidious achieved the goal of what many ancient Sith wanted and enslaved the galaxy and destroy the Jedi order.

Unfortunately this didn't last, as he was killed by his apprentice Darth Vader.

History:

Darth Sidious was born as Sheev Palpatine of Naboo during 84 BBY, He initially wanted to be a bike racer, but then he was seduced to the dark side by Darth Plaugueis and was indoctrinated into the Sith at the age of 17.

For many years he trained as a Sith lord while simultaneously studying about politics, becoming a politician due to the grand plan. At some point he went to Dathomir where he took a young boy from his mother on the begging of her to take him so he doesn't become a slave and he trained him as an Assassins.

He was elected Chancellor at 41 BBY after the battle of Naboo, something he staged to get into power, at the cost of his apprentice, he also Killed Plaugueis during this time and finally became Dark lord of the Sith.

Over the next 10 years he prepared himself for the clone wars he was to start while also grooming Anakin. He recruited former Jedi Dooku and Dooku became his Apprentice Darth Tyranus.

During the clone wars he didn't do that much as a Sith except ask for reports form Dooku. During Nihilus night he stopped Dooku from attacking anybody during that night since of it's significance to the Sith.

During the end of the clone wars he was captured by Grievous but was saved by Anakin and Kenobi. He took advantage of Anakin worry about Padme death from birth and managed to sway him to the dark side after the Jedi try to attack him.

He then initiate order 66 and started the Jedi Purge he envisioned. He declared that the Jedi Attacked him in the senate and after great applause he turned the republic into the Galactic Empire.

He was Attacked by Yoda two days later but he defeated his Jedi counterpart in combat, forcing Yoda to leave. He also saved Anakin from death and put him in an armour made with Sith Alchemy.

He Ruled over his empire with an Iron fist but wasn't that bad, He didn't keep anti-slavery laws but he did enslave the wookies since he believed that Wookies weren't sentient. He also formed the Inquisitors to hunt down Jedi survivors while making the Bogan-troopers for replacing Jedi in their peacekeeping jobs.

for many years he wasn't  challenged, becoming an even more powerful Sith lord, however that was to change. His first true threat to his rule was the self-trained Dark lord of the Sith Darth Olmec, who insurgence nearly concurred his empire(without much of an army either)  if it wasn't for him being killed by Darth Vader.

The second threat was the Rebel alliance who started to become a true threat from 4 BBY when the Galactic civil war started. They even became more of a threat to his rule after they blow up the Death star.

He had another problem, he was growing to old and needed an successor to his thrown, one that wasn't Vader. He saw hope in the young Luke Skywalker and tried to use Vader to seduce him to the dark side.

Unfortunately even he would die, it was at the peak of him winning, the rebel fleet was being destroyed and Luke defeated Vader when Luke refused to kill him. In fury he zapped the boy in force lightning but Vader saved his son out a fatherly spirit and was threw into the reactor core of the death star, killing him and ending his terror.

Legacy:

He would be feared through out history as the emperor of the republic while any future Sith revered him as one of the greatest and most successful Sith lord ever. Though he wasn't the strongest.

He would not die without a child though, who also had a child as Rey Palpatine, who unlike her grandfather would become a great Jedi instead a terrific Sith lord.

Notes:

He's based on the Canon Character of the same name.

In continuity, the main reason that I replaced Sidious with Krayt for Rey big bad guy is because I was never a fan of Sidious resurrecting himself, to be fair I would have seen ROTS as the best film in the sequels if it was another Sith lord, Like Darth Plaugueis.