Darth Ruyn was an aged male Twi'lek Sith Lord who served Dark Lord of the Sith Darth Krayt and his One Sith. He instructed several apprentices in the Sith arts, including the female Twi'lek Darth Talon. In 137 ABY, seven years after Krayt deposed Galactic Emperor Roan Fel and established his Sith Empire on the galaxy, Ruyn completed Talon's Sith training and presented his apprentice to Lord Krayt. As Talon's final test, Krayt commanded her to kill Ruyn, whom she respected and cared for, but task she nevertheless completed without hesitation.
Old at the time of the conflict between the Sith-backed Fel Empire and the Galactic Alliance beginning 127 ABY—known as the Sith-Imperial War—the male Twi'lek Darth Ruyn was a Sith Lord and member of the One Sith Order under the direction of Dark Lord of the Sith Darth Krayt. Ruyn trained Sith apprentices in the ways of dark side of the Force, the last of which was fellow Twi'lek Darth Talon. While teaching his apprentice, Ruyn would often take her to the Sith Art Chamber, within the Temple of the Sith on Coruscant, as he found it conducive to meditation.
In 137 ABY, seven years after Krayt deposed Galactic Emperor Roan Fel and established his Sith Empire at the end of the Sith-Imperial War, Ruyn completed Talon's training and presented his apprentice to Krayt. The Emperor gave Talon one final test: to kill her Master, whom she respected and cared for. Talon carried out Krayt's order without hesitation, beheading Ruyn with a sweep of her lightsaber. Having passed the test, Talon was given her Sith name and promoted to the position of Krayt's Hand—his own personal assassin.
Talon would later reflect that had Ruyn not deemed her ready, he would have blocked her attack. Instead, he died so that she could touch the dark side and complete her training.