Bane's lightsaber instructor was a Twi'lek blademaster named Kas'im.
After Darth Bane turned against the Brotherhood of Darkness, he faced Kas'im in a lightsaber duel on Lehon.
Bane was forced to admit that Kas'im was still a superior swordsman, but Bane by this time was far stronger in the Force and that was how he defeated his old teacher.