This is a summary of the first plotline of Celestial Divide, the important story events can be found
Here
Dark rumors have been running rampant in Equestria of a cult operating within its borders, trying to summon an ancient deity known as Eclipse to overthrow the Princesses. Monsters have been becoming very restless and bold during this time, often wandering out of the Everfree Forest and attacking innocent ponies. But when all eyes were turned towards Celestia for guidance, Celestia left. Taking the Elements of Harmony, the Princess of the Sun left on undisclosed business towards the Diamond Dog territories, leaving her sister, Princess Luna in charge.
Though the Princess of the Night was reluctant to take the reigns to the kingdom, she took leadership, heeding her sister's warning words about the danger Eclipse posed. Everything seemed to be merely rumors until a monster that came to be called the Spider Abomination appeared in the Everfree Forest, terrorizing Hoofington. Springing into action, Luna quickly dispatched a group of volunteers to dispose of the monster.
However, there was more than just a monster in the woods. The volunteer group, led by Ranger, Ponyville's militia Captain, encountered another group of ponies sent from the rumored Cult, led by the sorcerer Trixie and a three way battle ensued between the Monster, the Cult and the Equestrians. The Cultists sought to capture the monster alive and examine it, while the Equestrians did all in their power to kill it. In the end, the monster fell dead, not before killing one of the Cultists, later recognized as Ace Brighthorn from Canterlot.
Now more concerned about the Cult, Equestrian forces were ordered to stop it where they could find it. Soon enough, they found more than they bargained for. In Whitetail Woods a mysterious channeling ritual was taking place, sending a beacon of light into the Celestial Divide (Outer Space). Many a curious ponies arrived on the scene and demanded explanation, which the cultists were not willing to give. A fight ensued, joined by Equestrian soldiers, costing the lives of the cultist Pegasus Wildfire and the Equestrian civilian Quillson Oatswrite, who drowned in a canal of water, trying to swim towards the center of the ritual.
The Ritual was finished and the cult disappeared in a great splash of water, leaving the Equestrians baffled, but better prepared.
The Second Ritual took place in the sky, but this time Equestria was prepared. The Stars and a Pegasi assault platoon were immediately deployed, managing after a prolonged battle to interrupt the ritual before it was finished, sending the Cult fleeing back to their base in the Froggy Bottom Bog.
Now aware of the Cultists location, Princess Luna, who had received correspondence from their leader, Red Charge, decided to attack. Gathering as much intelligence about the Cult as they could, the Equestrian Army marched into the bog, preparing to stomp out the cult permanently.
Princess Luna and the military council decided on a strategic plan, sending three strike teams to destroy key targets behind enemy lines, while using the main army as a distraction.
Princess Luna was surprised to find that though the Cult forces were fighting, they have not killed a single Equestrian soldier, and healed those that were hurt, their intention was to delay, not harm. This took the fight out of her own troops, having to fight their own was bad enough, even worse was when the enemy was not trying to kill them.
The Strike Teams (Codenamed: Phoenix, Thunderbird and Moonbird) were to take down the Cult's massive Water Golem, spewing out clouds and projectiles made of concentrated Poison Joke concocted by Trixie and the leader of the Cult's mages (The Red Mages), Watermark. This mission was assigned to Team Phoenix, who failed miserably due to unforeseen circumstances.
Team Thunderbird was to take down the Cult's mage battery firing artillery stun spells at the Equestrian Army, they succeeded with the help of Valkyrie Razorwing, leader of the Stars, but were then faced with Whitefire, the lieutenant of the Red Mages and were forced to retreat. Whitefire by herself could not hold back the Equestrian Army, which kept advancing.
Team Moonbird was in charge of infiltrating the Cult's base structure, an ancient pyramid, and take down Watermark and Red Charge themselves, thus putting a stop to the Final Ritual and the summoning of Eclipse.
Unfortunately, Team Moonbird was unable to complete its mission, Eclipse was already summoned long before, the rituals, as Watermark revealed, were rituals meant to control Eclipse and diminish his power. Because the second ritual failed, Red Charge was not able to control Eclipse, and as the monster arrived it began to wreak havoc on the land.
With combined forces and after a long struggle, the Equestrian and the Cultist armies managed to kill Eclipse, ending his rampage forever and for the first time since the crisis began, Red Charge and Princess Luna could talk face to face.
It was revealed that Princess Celestia's warnings were unfounded, Red Charge had attempted before to contact Princess Luna but was denied, and his correspondence was misunderstood. The Cult's intention all along to summon Eclipse was to help curb the tide of monsters now rising from the Everfree, not to overtake Equestria. Realizing now that Red Charge was not responsible for the restlessness of the monsters a peace was mediated between the sides. Red Charge, revealed to be Silver Dawn, a unicorn once under the tutelage of the great Destrier, would become the new Royal Advisor, and the Cult would be cleared of the charges against it.
But the two sides had to wonder why Celestia was so intent on labeling Red Charge as a criminal with no evidence indicating as such…
