Chapter 1: Blank SlateShrine of Trials
The Dust Gigas is one of the rare Fiends in Mobius Final Fantasy. It first appears in Chapter 1: Blank Slate, and shows up on occasion in Act 1.
However, it did make a dramatic appearance in a much larger - and stronger - form during the Underground Coliseum event, around the Act 2 Chapter 3 timeframe.
Underground Coliseum:
Dust Gigas can appear as an enemy in the following locations: