Phobos

Phobos was a Sith Lord who was considered ancient by the year 35 ABY. The Sith Eternal, a Sith cult operating at that time, adopted the tradition of naming its Sith trooper units after ancient Sith.

Following that practice, the Phobos Legion was named after them. Since the history of the Sith largely remained a secret to the rest of the galaxy, only members of the Sith Eternal were aware of the significance of the unit's name.

Text and facts from the “Phobos” article on Wookieepedia, licensed under CC BY-SA 3.0. Modified for this site.