Alatar was a Maia of Valinor, his name meaning "after-comer" (last to be chosen to be an Istar) which could suggest he was renamed as a title (a note on this is that he had no other name in Endor, see below), designated by the Valar to travel to Endor (Middle-earth) as one of the three Istari (wizards, lit. "knowers"), whose mission was to aid the free peoples against the rising evil of Sauron. They were to be invested as kind old men and give advice to the peoples, not take direct action against the evil lords, as those who abided in Middle-earth were left to their own destiny.
But the original three of the Istari (Curumo*, Olorin and Alatar) became five, as Aiwendil (Radagast) was added by Yavanna and taken as a companion by Curumo, and Alatar took Pallando 'as a friend'.
Upon his arrival to Middle-earth, around 1000 TA, Pallando and
himself were dressed in sea-blue, which gained them the title of Ithryn
Luin, the Blue Wizards. They went together to the far east and
aid the remaining peoples of the King's men of Numenor as the Haradrim
and Easterlings. Saruman went with them at first but he then came back
to the west alone.
About their fate J.R.R. Tolkien tells us:
References: Silmarillion, Letters of J.R.R. Tolkien , Unfinished Tales