Bengel Morr first appears in the Jedi Knight storyline on Tython. Introduced as the player character’s first rival, the blue Nautolan actually has quite the tragic backstory.

Bengel Morr was Jedi Master Orgus Din’s last apprentice, and the reason why Master Din never wanted to train another Padawan again until he accepted the Jedi Knight as his apprentice. Both Bengel and Orgus had been present during Darth Malgus’ invasion of the Jedi Temple during the Sacking of Coruscant, however their perception of the event would change both forever.

While Orgus Din worked alongside Havoc Squad and fought against Darth Angral at the Senate Tower, Bengel Morr himself was trapped within the ruins of the Jedi Temple. Managing to only barely survive against the Sith attack, Morr crawled out of the ruins as a shell of his former self, suffering serious mental trauma from the event.

Watching further as the Jedi Order and Republic would go on to sign the Treaty of Coruscant, Bengel Morr was enraged by all that he had been raised to stand for. Believing the Republic and Jedi to have become hollow, weak and corrupt versions of themselves, he fell deeply into the Dark Side of the Force and recognized his place as a Dark Jedi.
As the Jedi retreated from Coruscant to Tython, Bengel Morr would spend the next decade in the wilderness of Tython, making alliances with the Flesh Raiders and corrupting the force-sensitive Callef into becoming his Padawan.

All of his work would culminate in the Battle of Tython, where Bengel Morr’s apprentice Callef woul lead an early Flesh Raider attack on the Gnarls in an attempt to destabilize the Jedi and kill many young Padawans and Knights before being stopped by a fledgeling Jedi Knight and Consular.
After the Attack on the Gnarls failed and Callef was defeated, Bengel himself would manipulate the Twi’leks of Kalikori Village in handing over Orgus Din and his new apprentice, the Jedi Knight, to the Flesh Raiders.

The Battle of Tython would then ensue, with the Jedi Knight rescuing Orgus Din and defeating Bengel Morr and the Flesh Raiders.

The Jedi Knight would set Bengel Morr on one of three potential paths:

1. Return to the Light Side and seek redemption
2. Remain in the Dark Side and flee Tython
3. Kill Bengel Morr to prevent this from happening again

If the Jedi Knight chose to turn Bengel Morr to the Light Side once more, Bengel would undergo years of therapy and redemption with the Jedi, culminating in joining the Liberation of Corellia from the Empire’s control during Act 3.

If the Jedi Knight told Bengel to remain within the Dark Sie and flee Tython, Bengel would flee the planet, periodically send the Jedi Knight secret messages stored aboard the Knight’s personal ship. These messages showed Morr becoming increasingly mentally unstable over the next several years.

Finally, if the Jedi Knight decides to kill Bengel Morr, he accepts his fate, nodding as the Jedi tells him to accept destiny.

Sources:
Star Wars: The Old Republic Encyclopedia 
All 3 Acts Dark Side – Jedi Knight Origin Story
The Old Republic: Threat of Peace – Comic