"The Emperor promised Angron a legion made in his image, limitless power, and life-times spent perfecting the Art of Conquest. But, to his surprise, Angron refused. He chose instead to die amongst his comrades while fighting his oppressors. Reluctantly, the Emperor returned to his flagship above.
Yet just as the battle was about to begin, the Emperor teleported Angron against his will back up to the fleet. He could only watch in anguish as those he regarded as his brothers and comrades were quickly annihilated." From: [link]
The main reference was Mr. Blanche's artwork. And.. I must admit I feel some sorry to Angron despite he is my most unfavourite primarch.