Complex sentence: In the sequel series, Fire Lord Zuko aided Avatar Aang in the creation of the United Republic of Nations after the Hundred Year War from the Fire Nation colonies within the Earth Kingdom and is the grandfather of General Iroh II of the United Forces.
Split into simpler sentences: In the sequel series, it is revealed that Fire Lord Zuko and Avatar Aang created of the United Republic of Nations after the Hundred Year War from the Fire Nation colonies within the Earth Kingdom. He is also the grandfather of General Iroh II of the United Forces.