Mandalore the Indomitable is a Taung warlord who leads the Mandalorian Crusaders before and during the Great Sith War. In 3996 BBY, judging the Empress Teta system weakened, he invaded it — and lost a personal duel to Ulic Qel-Droma, then commanding the Krath military.
His response is the whole culture in one gesture: he swore a personal oath to the man who beat him and pledged the Crusaders to the Sith cause. He died on Dxun in 3996 BBY, and the mask was picked up by an unnamed Taung soldier who would eventually be identified as Mandalore the Ultimate. A widely reproduced illustration in the Knights of the Old Republic Campaign Guide mixes the two of them up, and The Complete Star Wars Encyclopedia repeats the error — a good reason to check a caption twice.
Catalogue entry. This figure has a verified record here rather than a full write-up — the encyclopedia labels its own depth instead of padding it.