It is not known how the Druids came to join the Empire.
All of the Druids wear long brown robes that conceal much of their bodies, usually exposing only their hands. Aside from Haggar, all known Druids appear to wear white masks on their faces that make it appear as though they have more than two eyes.
In battle, all Druids possess energy manipulation abilities, can move fast and teleport, and Haggar is known for her abilities in casting illusions. They are capable of refining raw Quintessence into fuel for the Galra Empire. Druids are also effective as interrogators, and in experimentation with robotic elements to enhance soldiers and create Robeasts.
Zarkon also uses the Druids' magic to enhance the link he has with the Black Lion, being its former Paladin, allowing him to track Team Voltron anywhere, and take control of it at close range. Since Shiro has deepened his bond with the Black Lion, it has weakened his connection.
The white, bird-like masks worn by Galra druids bear a passing resemblance to those worn by plague doctors in 17th century Europe.
- ...which is very interesting, seeing how Shiro referred to the Galra as "These monsters spread like a plague throughout the galaxy," in the very first episode (The Rise of Voltron).