He was once married, but has been a widower for many years. His wife died before his rise to power.
He was one of the first to realize that the Last Battle was approaching but misinterpreted that he would be the leader of the forces of good and that they would all be under the banner of the Children of Light. He sent Geofram Bornhald to the Almoth Plain with a legion of Children of the Light. There he tried to put that area under Whitecloak control. From there he could have easier access to capture Arad Doman and Tarabon, and put them under his control. He later sent Padan Fain, under the command of Dain Bornhald, to the Two Rivers.
He committed treason by sending men to protect the Dragon Reborn in order for him to create chaos which Pedron would capitalize on to make his dream reality. He also gathered all other Children of the Light back to Amadicia.
He entered the Royal Palace of Amadicia to place Morgase Trakand under his protection. He promised to help Morgase take back Andor with his help. He also told her the news of Rahvin (Lord Gaebril) dying at the hands of the "false" Dragon, Rand al'Thor. This was all part of his plan to add Andor to his rule. She finally agreed to an alliance with Niall allowing the Children of the Light to be immune to Andor's laws when they finally ride into Andor. However he held back his invasion and also kept his men from attacking the Salidar Aes Sedai.
He may have been bound to the wheel at his death, from indicators in ACOS Prologue. It describes Niall seeing "grey mist" and leading a charge, in addition to his memory fading. Heroes of the Horn of Valere can be new made from men and women who show uncommon heroism, according to Artur Paendrag Tanreall. Niall's status as a Great Captain and one of the most respected and feared men of the Third Age leads some to think that he will eventually reappear bound to the Wheel.