There is a brief mention of Darth Sion in Star Wars: The Old Republic. A group of Kel Dor people who are part of a tradition called the Baran Do worshipped Sion and gave offerings to him. You find these as a player and can take the offerings for yourself. You can also buy Darth Sion’s armor on the Cartel Market. However, the armor is shaped after Sion’s DLC character design in Star Wars: The Force Unleashed, and not his design from KOTOR 2.