Your Jedi Knight will get the mission "Weapon of the Jedi" after completing the mission "Dark Temptations".

Open your mission log:

Weapon of the Jedi
Master Orgus Din is being held prisoner by Bengel Morr somewhere near the ancient Forge. T7's scans located a likely holding area inside the Forge entrance along the way to the Forge. Using T7's scans, you must search the area in the Forge entrance along the path to the ancient Forge and find where Master Orgus is being held.

Your task is to investigate the forge entrance. Head south from the Jedi Temple, past the Kaleth Ruins and into the Waterfall Cave. Inside Waterfall Cave, defeat Flesh Raiders and Reprogrammed Droids for the Bonus Mission "Remnants Invasion". Exit the cave to the south, reach the Forge Remnants. Climb the stairs and keep going south towards the Forge entrance. There is a boss there, Grand Chieftain Keshk, a Level 9 Warlord. Defeat him and keep going. As you get closer to the entrance, T7 will warn you that it is detecting hostile life forms and to be careful.

Go into the Forge Entrance. Defeat 15 Flesh Raiders Feral Adepts for the bonus mission "Forge Ahead". Master Orgus is not in the cave, but many Flesh Raiders were. Continue heading towards the ancient Forge. As you approach the Forge, you see Bengel Morr standing near the steps of the Forge, holding Master Orgus Din Prisoner. Confront him.

Bengel Morr strikes Master Orgus Din and turns to you. He tells you he sensed you coming. Your fear betrayed you. You are weak, like his old master and you will fall with him. Your dialog options will be to tell you why he hates you (LIGHT), or tell him that you will stop him (LIGHT), or tell him "Enough" [ATTACK] (DARK)

Dark choices will also decrease T7's affection. Affection affects how quickly companions craft items and do crew skill missions. It also affects the results of those missions.

If you ask Bengel why he hates you, he tells you that he does not hate Jedi - but rather the sick, spineless cowards that have taken their place. The traitors who let the Sith slaughter millions. He escaped the destruction on Coruscant to see his own Master surrender to the Sith. You now get new dialog options, "We'll never give up" (NEUTRAL), "This is about Revenge" (NEUTRAL), "Then he deserves death." (DARK).

You will enter a battle with Bengel Morr. This is a BOSS FIGHT. Begel Morr is tough to defeat. First get rid of the Flesh Raider Adept Bodyguards before turning your attention to Bengel. Make sure you have some heals ready. You might die a few times during this fight, thats allright. If you die, have a medical probe revive you and move back. Heal yourself fully and apply all buffs to you before challenging him again. Use Slash whenever you have enough focus.

After you defeat Bengel Morr, speak to him. He will be shocked that you have won, and wonders why has the Force guided him to this defeat. He tells you that you are stronger then any Jedi he has ever known, you could redeem the order and destroy the Sith. He tells you that you are the weapon he came to forge. If you threaten to kill him, he tells you that you don't have to strike him down. He is your servant now. He has spent a lifetime learning the paths to power. That knowledge is now yours. He wants you to let him leave Tython ans prepare the galaxy for your ascendance. He will deliver the gifts you need to claim your destiny.

Your dialog options now will be to "Rest Now [Knock him Out" (LIGHT), agree to the deal (DARK) or tell him no thanks and kill him (DARK). 

If you tell him that you have a deal, you ask him to give you the things you will need to defeat your enemies, and you will let him live. His first gift lies inside the Forge, where Force users created their lightsabers millennia ago. He has learned their forgotten ways, discovered how to create a truly powerful weapon. He shows you how. He tells you to use his knowledge to make your lightsaber, and the Force will be with you. He leaves, telling you he will find you when the time is right. T7 complains that the decision of  letting Bengel Morr go is against Jedi Council orders. You tell it that what the Council doesn't know won't hurt it. Then you erase all events from its memory logs since entering the Forge. He asks you if you saved Master Orgus and defeated Bengel Morr. Tell him that he was shot.

Speak to Master Orgus who is regaining consciousness. He thanks you for saving his life. If you knocked Bengel out, you tell him that he is resting quietly. If you made a deal with Bengel, you tell him that you fought Bengel Morr and he fell into the Ravine and died.

Orgus tells you that the horrors Bengel witnessed on Coruscant broke him, destoyed the gentle Padawan he trained. You, however, faced a challenge beyond any trial Orgus could assign. There is nothing more the Master can teach you. All that's left is for you to enter the Forge and construct your own lightsaber.

Orgus tells you he will return to the Jedi Temple and organize a search party for Bengel's body (or to take him there to get him some help, depends on your choices), and to meet him there when you are ready. Your task now is to construct your lightsaber. Climb up to the Ancient Forge and interact with it. This is the ancient Forge, where millennia of Jedi have constructed their lightsabers. You can make a Jedi Lightsaber (NEUTRAL) or if you made a deal with Bengel earlier, you can use Bengel Morr's knowledge to make a Dark Jedi Lightsaber (DARK).

SWTOR Lightsaber Creation

You have now created your own lightsaber. But a deadly beast from the Forge's depths heard your labors and has come seeking prey. Defend yourself against the monstrous creature at the Forge. Defeating the Forge Guardian completes the bonus mission "Putting your new weapon to use". This is a boss and is very tough even with the lightsaber, so try a few times to kill him, and if you don't succeed just proceed with the rest of the mission.

Your task now is to report to the Jedi Council. Return to the Jedi Temple and head to the Council chamber on the 2nd floor. Orgus is debriefing the council. The Flesh Raiders are still a threat, but without Bengel's leadership, they'll be scattered and ill-equipped. Master Satele Shan thanks your for saving the temple. She ask you about your opinion regarding the recent developments in the Twi'lek settlement. You can tell her that the Twi'leks need our support (LIGHT), that we should take control (NEUTRAL) or that we should force them out of Tython (DARK).

Orgus asks Master Satele Shan to make you a Jedi Knight, and she confers on you the title. You no longer are a Padawan, but are now a Jedi Knight. May the Force be with you. T7 requests permanent assignment to assist Jedi. Say that it's a fine idea. Satele agrees.

Master Orgus reports he is getting a priority signal from Coruscant. It's master Kiwiiks. Master Bela Kiwiiks reports that she has discovered a situation requiring further assistance. The dark presence is strongest there. Something malignant works in the shadows of the capital. We must find it. Master Satele instructs you and Master Orgus to join Makster Kiwiiks on Coruscant. Kiwiiks tells you to come to the Senate tower as soon as you can. Orgus tells you to complete any unfinished business you have and take a shuttle to Coruscant.

This completes the mission "Weapon of the Jedi" and begins the next mission, "Seeking Darkness".