During the events of Shadow of Revan your player character, with the help of Theron and Lana, uncover an undersea experimental base on Manaan. This scientific station (run by Gorima the Selkath) was defended by the Order of Shasa, a cult of force weilding Selkath that swore loyalty to the protection of Manaan from the influence of outsiders.

Why?

During the game events of KOTOR 1 whilst on Manaan, Revan himself took time to aid a worried Selkath father about the whereabouts of his daughter ‘Shasa’ a force sensitive.
As it turns out, the local Sith embassy on Manaan had abducted Shasa and attempted to influence her mind with the Dark Side of the force in order to enact atrocities via her acting as a proxy. The Sith planned to wipe out the Selkath government so that they could take control of the planet and its valuable Kolto resources.

This however did not go to plan as Revan uncovered the plot of the Sith and frees Shasa from their control. After the player departs on their journey, Shasa would go on to create the Order of Shasa, a force-religion of Selkath that were loyal to not the Jedi or Sith, but the protection of their kind and home.

Circling back to SWTOR:

In the build up to the Revanite War during Shadow of Revan, Colonel Darok and Darth Arkous (under the leadership of Revan) recontacted Manaan and made ties with Gorima, a Selkath Scientist that agreed to use Revan’s recovered Rakatan technology in order to create an ‘Infinite Army Prototype’ of cyborg soldiers.
However doing so under the nose of the galaxy would be risky and may draw unwanted attention from any faction.

Indebted to the Revanites as Revan had freed their leader Shasa (thus enabling the creation of their Order) over 350 years ago, the Order of Shasa served as the defence platform and force of the underwater facility, dedicated at all costs (even death) to protect Revan’s plan on their home world from any outsider, Republic or Empire.

Even though the underwater base collapsed during the fighting, the Order of Shasa would go on to exist well through to the Galactic Civil War era where they would eventually all be tricked by Darth Vader into serving the Emperor.

Sources:
Knights of the Old Republic 1,
Star Wars the Old Republic (Shadow of Revan),
SWTOR.com Developer Blog: Since KOTOR: Manaan