The Grand Marshall was the leader of the Ice Warriors.
He was similar in appearance to Ice Lord Slaar, although he wore a more ornate helmet. He spoke normally (rather than the hoarse loud whispers spoken by some Ice Warriors), as he was in 'his own atmosphere'.
Eventually he was killed, along with the rest of the Ice Warrior fleet, when Slaar's homing beacon was hijacked by the Doctor, sending them crashing into the sol. (DW: The Seeds of Death)