"Altar of Mortis" is the sixteenth episode of the Star Wars: The Clone Wars television series' third season. It aired on February 4, 2011.
"Before the Jedi can leave Mortis, the Son takes Ahsoka captive in an attempt to entice Anakin into joining him to use their combined strength to overpower his Father and Sister. To this end, the Son casts Ahsoka under the spell of the dark side.
Meanwhile, the Father attempts to stave off a disastrous showdown between his children and maintain the Force's increasingly precarious balance on the planet. Fearing that the Son may be unstoppable, the Daughter does the forbidden by taking Obi-Wan Kenobi to the Altar of Mortis, wherein is kept the Dagger of Mortis, a weapon capable of killing a Force wielder. The Son steals the weapon, and attempts to use it against his Father to steal his power and end his rule, but the Daughter sacrifices herself, placing herself in front of the blade."
As Anakin, Obi-Wan, and Ahsoka are about to leave Mortis' world, Anakin, while resting from this morning's ordeal, has a disturbing dream in which he finds himself back in the planet. What appears to be a mirror image of himself approaches him, only to reveal itself as Son, who asks Anakin to join him on the dark side of the Force and restore peace and balance in the galaxy. When Anakin refuses and turns away, Son turns into a monster and attacks him, jerking him awake. Ahsoka walks into the room meanwhile. Suddenly the ship tilts and starts rocking wildly. But just as Anakin prepares to join Obi-Wan at the controls, Son suddenly appears behind them, kidnaps Ahsoka and jumps out of the shuttle. He shifts into his gargoyle form and flies off with Ahsoka in his talons.
Running into the cockpit, Anakin took over and chased after Son into a canyon of mist-shrouded rock spires, but Son puts on a burst of speed and disappears into the mist. Anakin flies even faster and instead comes upon a strange tower supporting a huge glowing sphere. Just narrowly avoiding a collision with the structure, Anakin is unable to control the shuttle, and it crash-lands back on the planet, in an area dominated by darkness. In the distance they can both clearly see the tower with the glowing sphere on top of it; Obi-Wan assumes that's where the Son had Ahsoka. Claiming it was his fault the Son had taken Ahsoka, Anakin prepares to go for the tower, but Obi-Wan holds him back, desperately cautioning him not to let himself get lured into an obvious trap lest it would yield catastrophic consequences. He suggest consulting Father about how to proceed, but Anakin nevertheless leaves for the tower. While Obi-Wan ponders how to reach the Father in the first place, he suddenly finds himself very close to the monastery. Inside her prison cell within the tower, Ahsoka tries to free herself, she hears a voice and then she finds herself in the company of a creature which addresses her innermost doubts and fears. It questions where her Master is and claims that he won't come. Ahsoka snaps that Anakin will rescue her. It releases her from her chains. It tries to convince her to forget her Master, but seeing that she will not willingly turn, it bites her in the arm. Clutching her arm, Ahsoka collapses on the floor, the creature morphs back into its true form, the Son; his bite now infects Ahsoka with the dark side of the Force, and she undergoes a horrifying change.
Son thereafter joins his father and sister; Father expresses worry about his son's descent into the Dark Side and implores him to let go of it, whereupon Son lets loose with his anger and blasts Father with Force lightning just as Obi-Wan arrives. Daughter who had heard the fight ran to help her father. Upon Father's sickbed, Obi-Wan implores Daughter to assist him in stopping Son before he escapes the planet, but Daughter, saying it is not Son's fault as that is his nature, refuses to raise her hand against her brother. Upon his suggestion that she and Anakin should combine their powers to stop him, Daughter instead leads him to a cavern which contains an altar holding a blade formed by pure Force, which Obi-Wan is meant to wield against her brother.
Meanwhile, Anakin manages to scale Son's tower and finds Ahsoka waiting for him. Unfortunately, he discovers that she is corrupted by the dark side; Son infused her with part of his own soul, bringing out her insecurities and resentments along with her other more aggressive tendencies. She leaves Anakin a message: either join the Son or he kills Ahsoka. When Anakin dismisses the threat she ignites her lightsaber and viciously attacks him. Obi-Wan and Daughter confront Son in his inner sanctum, but he manages to separate them by directing Obi-Wan into assisting Anakin. As Obi-Wan arrives Ahsoka turns and activates her shoto and begins to attack them both. During the fight Anakin turns to Obi-Wan knowing that they could not defend themselves for long without either harming Ahsoka or her eliminating them. Obi-Wan then pulls out the Mortis Dagger he received from the cave altar and suggests they free Ahsoka from the Son's influence. Ahsoka, angered and knowing exactly what they have, continues fighting the Jedi to get the blade from Obi-Wan. Meanwhile, Son and Daughter began a duel using their Force powers to overpower each other. They later began shifting in and out of their beast forms. Soon however, they were interrupted by the recovered Father, who hurls them among the Jedi. Son turns his anger against his father, who is unable to prevail against him in his still-weakened state. During the whole ordeal, Ahsoka got up and smirked as the Son uses lightning on the Father. Obi-Wan throws the dagger to Anakin, but Ahsoka dives, catches in mid-air, then runs up to the Son and hands it to him. Son thanks her and claims she is no longer useful to him anymore. Son taps her on the forehead, removing her life force and therefore killing her instantly. Her lifeless body collapses to the floor as her lightsaber rolls out of her limp hand. Enraged, Anakin lunges at the son, but is repelled by the Force. Son then turns and prepares to plunge the dagger into his father, but Daughter steps in front of her father, taking the blade instead, she is mortally wounded. Screaming with sudden anguish, Son flees the scene as Anakin rushes to Ahsoka's side, only to find her eyes completely rolled back.
With Daughter dying, the balance of the Force is upset, and the planet begins to descend into eternal darkness. Anakin, kneeling beside Ahsoka's lifeless body, begs Father to help her. Daughter uses Anakin as a conduit to channel the last of her life force into Ahsoka, removing the darkness within her, bringing her back to life. Ahsoka awakens, weak, but alive. Anakin hugs his apprentice and asks if she is alright, only to find out that she remembers nothing of what had happened to her. Father implores the Jedi to leave before Son can take their ship to escape the planet, and warns them that with the balance disrupted, the dark side and the Sith will gain the upper hand in the galaxy. Unable to do anything else, the three Jedi leave the mournful Father by the side of his daughter.