Mugen was orphaned at a young age after his parents were murdered by Tuigans from the Endless Wastes when the horde swept through Northwestern Kara-Tur in 1326. He was taken in by his Uncle, a master swordsman named Rigato Tachiro who owned a Samurai Dojo in Senita on the Islands of Kozakura. Rigato determined that his nephew was a very capable young man and put him to the task of learning the ways of the Samurai.
Mugen thrived on the rigorous training regime, excelling in his studies of Kendo, Zen & Bushido while also distinguishing himself in his studies of other cultures such as elves, dwarves & eastern humans. Mugen rose quickly to become one of Rigato's finest students. They became like father and son and on occasion discussed Mugen eventually taking over the Dojo.
One night, when he was nineteen years old, Rigato called Mugen to the Dojo's training yard. Mugen met his uncle with a warm smile. Rigato met his nephew with cold steel. The Dojo master attacked Mugen without explanation, intent on killing him, but Mugen was quicker, reacting to the threat with cold steel of his own. A flashing frenzy of swords, martial arts strikes and sweeps ensued with the closely fought combat finally ending with Mugen driving his Katana deep into Rigato's abdomen. With his last breath Rigato apologised to Mugen and told him that he had no choice, that it had to be this way.
That night Mugen fled the Dojo knowing that he would be accused of murdering his Uncle in cold blood and would face imprisonment or execution. He took work on a trading junk to get to the continent and then made his way across Faerun over the next six years, no longer a Samurai but a Ronin, a masterless Samurai, in search of meaning and purpose. In 1347 Mugen was working aboard the trading galleon "Redfish", when he arrived in the port town of Phlan, a town overun by monsters that was slowly being reclaimed by an alliance of merchants, priests & mercenaries.
Mugen joined a small band of adventurers who were journeying to a pyramid, upriver from Phlan, that was thought to be the source of the toxic sludge that had poisoned the river that ran down to the harbours. Less than an hour after entering the Pyramid the entire rest of the party had been killed by monsters and traps, leaving Mugen wandering the corridors alone and lost.