Main Timeline of Story Events

First Ones engage in war with the Horde under Horde Prime It is unclear exactly when this happened.

Mara discovers that the Heart of Etheria is a weapon with the potential for Universal Omnicide.   Because Mara does not want such a weapon to be used, she incites a rebellion against the leaders of the First Ones.   Because the rebellion is unable to dismantle the Heart using the Failsafe, Mara gives her life to hide the Sword of Protection and seal the planet in the dimension of Despondos.   Because it is separated from the wider universe, Etheria does not experience further contact with the Horde or the First Ones for centuries.   Because of her connection with First Ones Technology And/Or Etherian Magic, Mara's influence persists in as memories in the Beacon Temple   Because of Madame Razz's temporally disjointed consciousness, Mara is aware of Adora and leaves a message behind for her to find on her ship.

The First Ones planetary administration A.I. called Light Hope attempts to use limited portal capability to capture an infant with the potential to be She-Ra from a surviving population of First Ones. It is unclear why she did this now, whether there had ever been or would ever be other candidates. Hordak detects the portal and captures the infant Adora before Light Hope.

Because of the cataclysmic weather, the Horde is able to mount an assault on Bright Moon in Force. Because of Catra's tactically sound battle-plan, the Horde has an overwhelming initial advantage. Angela is tied up defending the moonstone, which Scorpia keeps Glimmer from reinforcing her. Catra keeps Adora busy while the rank and file distract Bow, Spinnerella, and Netossa.
The battle nearly ends in catastrophe, but Mermista, Perfuma, Frosta, and Sea Hawk arrive to throw back the Horde. The Horde is defeated with an overwhelming force of magic.

The Events of S3E5-6 a lot happens.

Because she follows Razz around while the latter engages in Chronosthetic mental time-travel, Adora is able to locate a missing piece of Mara's ship. Because Bow has been working to repair the ship, the addition of the missing part reveals additional holographic logs of conversations between Mara and Light Hope which make it clear that Etheria contains a weapon built by the first ones and that Mara was opposed to using it. Mara "becoming corrupted" was Light Hope's explanation for Mara's opposition to the First Ones Leadership's plan.

Because Light Hope gave Glimmer instructions on what needed to be done to balance the planet, Glimmer is able to get Scorpia to reconnect with the Black Garnet. Because Scorpia is reconnected to the Black Garnet, Light Hope is able to safely transport Etheria back to the universe. She then begins preparing the firing sequence. Because she does not want the weapon to fire under any circumstances, Adora shatters the Sword of Protection, thus disabling the weapon. Adora learns that Light Hope was directly responsible for the portal the brought her to Etheria as an infant. The whole process had been a plan by Light Hope to be able to use the Heart of Etheria.   Because Etheria is back in the universe, Horde Prime finds it.

Because Entrapta must patch directly into the Horde's spire in order to jam the signals controlling the chipped people, the rebels attack the spire.
Because Entrapta's hacking does not go unnoticed, she is beamed aboard Horde Prime's ship, but manages to push her equipment free of the beam's influence first.
Because Scorpia and Mermista take the field, the rebel fighters are put on the defensive.
Because they became separated from Adora during her quest to reach the Heart, Glimmer and Bow take the field. Melog is with them because Catra stayed behind to chase down Adora. Glimmer goes to reinforce the fighters in the field while Bow goes to finish what Entrapta started. Bow takes Melog with him. Glimmer manages to turn the fight on the field. Sea Hawk manages to distract Mermista by flirting with her long enough for a sneak-attack to the chip to disable her.
Because Scorpia is now patrolling the spire, Bow has to rely on Melog's invisibility to evade her while he finishes Entrapta's work. Bow is able to disable the chips and override Horde Prime's network, though not without difficulty. Bow broadcasts a message to rally popular support. It's very effective.
Glimmer manages to turn the tide of battle, but it is nearly in vain when Micah appears. Glimmer fights her father one-on-one and ultimately prevails, curing him of his dark-magic affliction with energy beams that look strong enough to disintegrate concrete.
Because he's been trying to override the planet's systems and absorb the Heart of Etheria this whole time, Horde Prime is enraged that he's being so badly interfered with. He orders Hordak to execute Entrapta.
Because he has remembered that he loves Entrapta and is a person with his own value and values, Hordak defies Horde Prime's will and shoots Horde Prime before dropping him off a ledge. Horde Prime possesses Hordak's body and grabs Entrapta before teleporting to the planet's surface to announce that rather than hold omto the power of the Heart for himself, he will now simply destroy everything.
His plans for universal genocide are stopped by Adora.

Because Adora believes she has to do this alone, she parts ways with Glimmer and Bow in the tunnels beneath the First Ones fortress.
Because she has seen what Horde Prime is doing and how it's affecting the planet, Catra returns to camp to find Adora. She only finds Shadow Weaver and demands that the latter help her to reach Adora in time. Shadow Weaver grudgingly complies. The two of them reach the place where Bow and Glimmer became separated from Adora. The two of them pursue Adora deeper into the catacombs. Glimmer and Bow depart to join the battle.
Adora sees a memory of Mara. Adora promises Mara that her sacrifice won't be in vain. Mara says that she paid the price she did in the hopes that nobody would ever have to pay such a price for the fate of the universe ever again. Mara asks Adora what she wants from this when it's all over and tells her that she's worth more than what she can give to other people. Mara tells Adora that she deserves love too, saying that she's so close and not to give up yet.
Horde Prime finds Adora in the system. Because Horde Prime has taken control of the planetary systems, he sets a security-system monster to attack Adora.
Because Catra and Shadow Weaver catch up to Adora, Catra is able to keep the monster at bay while Adora and Shadow Weaver flee towards the heart.
Because Adora refuses to leave Catra to die, she turns back despite Shadow Weaver's protests.
Because Adora would not abandon Catra even under such dire circumstances, Shadow Weaver puts herself between Catra and the monster and commands Catra to take Adora and run. When Catra doesn't immediately obey, Shadow Weaver forces her out of the room with magic and then seals the exit behind her. Shadow Weaver implores Catra to make sure that Adora reaches the heart. She tells Catra that she's proud of her. Shadow Weaver gives her life to save Catra and Adora from the monster.
Because Adora has nearly succumbed to her injuries, Catra helps her walk the rest of the way to the Heart of Etheria.

Because she cannot transform into She-Ra, Adora realizes that she will need to give her life to disable the heart before Horde Prime can absorb it. Adora claims that Horde Prime's virus is keeping her from becoming She-Ra.
Because she thinks she's going to have to give her life for this cause, Adora asks Catra to get as far away as she can. Because Adora is still the only thing that Catra really values, she flatly refuses to leave and holds on to Adora while the latter prepares to use the failsafe.
Because she attempts to use the failsafe while Horde Prime is in the middle of absorbing the heart, Adora is incapacitated.
Because she falls partially unconscious in the chamber of the Heart of Etheria, Adora experiences a very vivid holograph/dream of what she wants to experience in her life after the war. She sees Catra, Glimmer, and Bow excitedly preparing for a ball. They appear to simply enjoy one another's company and the prospect of a good party.
Because it was at least partially related to the memory-holographic technology of the temple, Horde Prime is able to interfere with Adora's wish for her future. Horde Prime attempts to place some kind of system-control on Adora that restrains her.
Because Catra wants Adora to keep going and not give up, Catra tearfully confesses that she loves Adora and begs her to stay. Because Adora loves Catra, she is able to muster the fortitude to overcome the system-constrain, transform into She-Ra and deploy the failsafe.
Catra and Adora kiss.
Because the failsafe is used, the Heart of Etheria safely discharges its magic back into the planet. The biosphere is immediately turbocharged.
Because Horde Prime no longer controls the Heart, Adora as She-Ra is able to drive his consciousness out of Hordak's body.
Because the universe has been saved, there is much rejoicing.