The Darkening Wild
The Istari are Maiar who were given a specific purpose – to travel to Middle-Earth and help the Free Peoples resist the Enemy – certainly Sauron, but perhaps in earlier ages, even Morgoth.
The three main Wizards, Curunir, Olorin and Alatar, were sent specifically to inspire resistance in the Free Peoples. Aiwendil went with Curunir, though the two did not get along, and Pallando went with Alatar. Olorin went alone.