The second man to take the name Matthias, Matthias Kya II was a Moff in the reborn Empire, and eventually became an Admiral, fighting at the first and second Battle of Taris during the Sith-Imperial War. His claim to fame was when he was annointed 'Emperor' during the Force Cold War.
Quite late in life his mother. Cesca Kya had a son to fellow Imperial out of wedlock in 54 ABY. Naming him after the founder of the modern Kya family, Cesca signified the new beginning she had brought about. Inheriting his mothers zeal for the Empire, Matthias went onto become an Imperial diplomat, doing very little special. His son, Aden Kya II, born in 89 ABY, was Force sensitive, the first in the long (or at least acknowledged) family line, and Matthias immediately sent him to join the Imperial Knights.
That political act gave him the power to curry enough favour with the second Emperor to become a Moff of Orinda, the fortress world. Dabbling in military affairs from there, the instabilities leading to the Force Cold War enabled him to be given the rank of Admiral. However, even Matthias could not have foreseen the events of 100 ABY.
The reunification of the Jedi Order on Corellia signified that the cold war was about to enter a new phase. The name 'Darth Mikaru' pushed the GA and Empire towards war, and the Sith returned, launching terrorist strikes on key worlds of the Empire - destroying the Imperial Palace on Bastion. Suddenly the Empire was fraught with factions as the Emperor was declared dead, and a spate of assassinations were attributed to Matthias - who suddenly found that he was being positioned as the next Emperor.
A Sith known only as Dragon arrived, holding Matthias hostage and feeding him the words to turn the disappearance of the Emperor into an opportunity. Declaring himself Emperor, he took command of the Orinda fleet, starting the Imperial Civil Conflict. The Emperor resurfaced, but Matthias was forced to denounce the true Emperor as a clone, and the Battle of Orinda followed.
The Imperial Knights launched an assault, capturing Matthias' flagship. Escaping Dragons clutches, he assisted the Knights, redeeming himself. Returning the title of Emperor to its rightful owner with some embarassment, the Emperor was merciful and restated Kya as an Admiral - his loyalty assured.
On isolated Orinda - no longer isolated by geography, but by its fortress world mentality - Kya continued the serve the Empire, ignoring the milling tide of politics that threatened to ruin the Emperor. The Moff Council was growing more prominent, and Kya simply prepared for war. When it finally broke out in 127 ABY, he was ready for it.
Fending off attempts to undermine his position early on in the war, Kya coordinated with Grand Admiral Veed and launched a massive offensive on Taris, winning the First Battle of Taris six months into the war, Veed pressing the GA back and defeating them in detail at the Battle of Bogden. Kya watched with increasing concern as the GA rallied, and he requested Imperial Knights to stymy the Jedi when they eventually came back, his son joining the detail.
A few weeks into 128 ABY the GA advanced, and Kya was engaged with a fleet many times the size of his, and considerably more powerful. Trapped, Kya and his fleet was crushed by the GA and the Jedi, pushing his son towards the dark side. His legacy was minute - just another cog in the Empire.