Shandar was the King of the Rocket Men. He was married to Myrren.
He wanted to negotiate with the Decayed Master and so stole K9 Mark I to use as a guard dog to kill the Master if he betrayed him. The Master thought he was imbecilic for keeping the Fourth Doctor on board the Asteroid.
He was the fifth most wanted criminal in the galaxy, the first three being the Master, the Rani and the Terrible Zodin. (AUDIO: Requiem for the Rocket Men)