Story wise saint million is the actual boss of the desert area, with kondrieru being the rare variant.
Shambertin is a event boss that you could fight and spam essentially at any time through taking it's mission, it had different drops than saint million (almost always better) and was a harder fight than saint million, kondrieru had better drops and was stronger than both though.
Kondrieru, the rare variant, could show up instead of either saint million or even shambertin in it's event quest, making shambertin's quest (since desert treks could legitimately take a couple hours even with a good party on higher difficulties, while a shambertin run could be done in 30 minutes) the best method for farming kondrieru, which had a 1/5 chance of spawning if i remember right.
EDIT: looked it up, that IS shambertin's music, this is saint million's music: