Mandalore the Ultimate is a Taung, one of the last of that species to hold the title Mandalore, and the war leader who fights the Republic in the Mandalorian Wars from 3976 to 3960 BBY. His first published appearance is a single unnamed Taung soldier in the final issue of Tales of the Jedi — The Sith War, picking up the fallen mask of Mandalore the Indomitable. He was not given the name Mandalore the Ultimate until years afterwards, in a Star Wars Insider article on Mandalorian history; The New Essential Chronology later confirmed the two were the same figure.
The Neo-Crusaders
His project is survival. The Taung are dying out and the Mandalorians are humiliated after the Great Sith War, so he reorganises them as the Neo-Crusaders — standardised armour, absorbed recruits from conquered worlds, a culture rebuilt around war as an identity that can be adopted rather than inherited. That single decision is why Mandalorians in later fiction are a creed and not a species.
The war that made Revan
He picks off Outer Rim worlds while the Republic argues about whether to respond, and the eventual response — Revan defying the Jedi Council to lead the fight — produces the Jedi Civil War, the Sith Empire of Malak, and everything Knights of the Old Republic is about. He dies at Malachor V in 3960 BBY when the Mass Shadow Generator is detonated.
Where to find him
He is discussed constantly in both KOTOR games and appears in John Jackson Miller's Dark Horse series Knights of the Old Republic (2006–2010), which is set during his war and is by some distance the best account of it.