Yu-Gi-Oh! GX episode 138: "The Darkness Revealed"
Nero's grandfather is a an elderly man, one of the residents of a village in the the alternate dimension.
While hiding from the forces of The Supreme King in a cave, he and his grandson Nero are approached by Jim Crocodile Cook and Axel Brodie. Jim and Axel ask if they've seen Jaden Yuki, showing them a photograph of him. They have not, and tell him that if Jaden is a Duelist, he has likely be captured by The Supreme King's forces. Nero and his grandfather take Jim and Axel back to their village, where they introduce them to Burgundy, who initially mistakes them for members of the King's Army and knocks Jim out. Later, Jim and Axel leave the village to find The Supreme King. They find their trip to his keep a waste of time, finding only Kozaky there. They return to the village in time to see it destroyed as Burgundy urges them to flee. Jim sees Nero's Duel Disk on the ground, and realizes Nero and his grandfather have been killed. This is one of the final factors the pushes Jim into challenging the Supreme King.