He suffered wounds to his face during a battle on Alderaan. In a fight with Republic Commander, Jace Malcolm, Jace first shot blasts from a heavy gun at Malgus, burning his face. Later in the battle, Jace tackles Malgus while holding a grenade. The explosion from the grenade damaged the face of Malgus even further. After that battle, Darth Malgus had to wear a respirator mask to help him breathe. This tends to be a common thing in the Star Wars universe, and you can see the same thing happening with both Darth Malak and Darth Vader.