Indigo had been expecting something in front of him, not behind him. So he hadn't really been thinking that he should listen for something coming from behind. As a result, he didn't hear the footsteps until the suit of armor was almost upon him. But eventually he did hear something, and he whirled around to come face to face with his would be attacker. Wand out, he tried to remember which of the three spells would work on the suit of armor. If it was a typical suit of armor he would have some trouble, but this one was made of glass, which meant he just needed to perform the spell to revert it back to sand. Luckily, he had just done that spell so he knew exactly what to do. "Prima forma sabulo." He made a counterclockwise motion in the air and ended with a point at the suit of armour to revert it to sand.