Excerpt from "Galactic Military History: The Rise Of The Empire (Second Edition)" by Vilmax Locke:
During the dawn of Emperor Palpatine's "New Order", worlds that disagreed with the Emperor's view were either subjugated or completely destroyed unless they had something that would further Palpatine's goals. One such world of interest was Spart'a, an isolated world in the extreme outer rim. The citizens of Spart'a, led by the wise King Leo Ni'Das, were renowned for their physical fitness and prowess in battle, both attributes that Palpatine saw as beneficial to his new armed forces. Spart'a, however, disagreed with Palpatine's politics and attempted to withdraw from the newly born Galactic Empire. Papatine refused to allow them to govern themselves and set about capturing the planet through military means.
The campaign was a long and bloody one for the Empire. Due to the magnetic field of Spart'a, similiar to that of Jabiim, preventing repulsorlift drives from functioning properly, the Empire was unable to utilize speeders of any kind. The Imperial ground forces were forced to rely on tracked, wheeled, and walker driven vehicles only, most of which were too large to navigate the narrow, winding mountain passes leading to the heavily fortified Spart'an cities. The Imperial forces fought virtually every engagement on their feet as they were unable to move their heavily armed vehicles close enough to the action to have an effect.
The Empire lost almost a full third of their Clone forces trying to take Spart'a. It was eventually decided that Spart'a was a lost cause and would be destroyed from orbit. The Imperial forces in orbit rained down destruction on all of the Spart'an cities, destroying all but Thermop'ilea, the capital, which was protected from orbital bombardment by a massive force field. The Imperial ground forces were tasked with disabling the forcefield, located in an underground bunker accessible only through a narrow pass known locally as the "Hot Gates". The "Hot Gates" were protected on one side by a cliff dropping off straight into the deep oceans of Spart'a and a mountain wall on the other, making the pass far too narrow for anything but foot soldiers. King Ni'Das knew that the generator must not fall and took three hundred of his best warriors with him to guard the "Hot Gates".
The Imperial forces threw wave after wave of Clones against King Ni'Das' three hundred Spart'ans who held them off for over a week of constant battle. The Spart'ans even fortified their defenses with the armored bodies of the fallen Clones to taunt their Imperial attackers. It was only after a traitorous Spart'an Senator, bribed by promises of power from The Emperor, informed the Imperial military of a small pass that would allow Imperial troops to encircle the Spart'an defenders from above that the Empire gained the upper hand.
The brave Spart'ans were slaughtered, save for their King. King Ni'Das, heavily wounded, was taken to the Imperial command ship in orbit and forced to watch as his beloved people were wiped out in a bombardment that left once beautiful Spart'a reduced to a smoldering cinder. It is believed that King Ni'Das was executed afterwards in a show of force to any other worlds that would seek to defy Emperor Palpatine's edicts.
Excerpt from the personal journal (dated near the end of the Galactic Civil War) of Commander Wedge Antilles, Rebel hero:
"We received the most incredible surprise this afternoon while liberating an Imperial asteroid prison colony. In one of the oldest cells, rotting away in his own filth, we discovered none other than the legendary Spart'an leader, King Leo Ni'Das! It was believed that he was executed after Spart'a fell, but here he is, alive and in the flesh. Even though he is over seventy standard years old, he is in better physical condition that half of the troops in the Alliance. He even took up a blaster and helped us mop up any remaining Imperial resistance.
Apon return to the fleet, King Ni'Das cleaned himself up and met with our leaders. He wanted to fight alongside the forces of the Alliance and do his part to ensure that no other worlds share the fate of his lost Spart'a. Can you believe how amazing this is?"
Excerpt from the personal journal (dated nine years into the New Republic's existance) of Chancellor Leia Organa-Solo:
"General Leo Ni'Das, much like myself, hails from a world destroyed by the Empire. After the war, General Ni'Das set about recovering some of the Spart'an artifacts stolen from his world by the Imperial forces during their campaign on Spart'a. Some items ended up in museums across the galaxy as remnants of a civilization long gone, but others ended up in the personal collections of the ranking Imperials present during the camaign or their superiors. General Ni'Das' helmet and vibro-sword were sent to Emperor Palpatine himself as proof of Spar'a's demise and ended up in one of his off-world storehouses. They were recovered by Rogue Squadron along with a Spart'an spear when they fought off pirates attempting to pilfer the storehouse. The Spart'an artifacts have been returned to General Ni'Das and the other items are in the process of being returned to their worlds of origin.
General Ni'Das has taken to wearing his old helmet, along with a replica cape made for him when he was promoted to the rank of General, and carrying his old weapons in honor of is fallen people and their world. General Ni'Das currently teaches ground battle tactics at the academy and is training a highly advanced commando squad he is reportedly calling his "New Spartans"..."
General Leo Ni'Das
Parts:
Head- POTJ Senate Guard
Body/cape- POTF2 General Lando Calrissian
Hands- POTF2 Biggs Darklighter
Vibro-sword- G.I. Joe Spearhead v1
Spear- Chap Mei
My only beef is that I so used to seeing these incredible customs from you (painting/parts/mods) that something as simple as a head/hand swap doesn't seem as exciting. 
