Let me tell you a story...
It was a cold day in Runeterra for Jarvan Lightshield IV, Prince of Demacia. He had allied himself with Katarina Du Couteau and Jericho Swain against the Summoners of the Institute of War. Garen was even now moving forward with the Dauntless Vanguard to begin his assault at the entry of the Institute of War, which Jarvan knew would be guarded by a host of the small, golem-like minions that the Summoners could bring forth in massive numbers.
At first Jarvan had been worried that he, and others like him, would be summoned to fight against his own army, but Leblanc had found the solution to that problem. By contaminating and warping small shards from a damaged Nexus she had managed to create crystals that would protect them from being used as little more than puppets by the Summoners. There were enough to outfit the leaders of Demacia and Noxus against this insidious mental intrusion, and the other champions, who had agreed that they no longer desired to be bound to the power of the Summoners, were on the shores of Ionia protected by the largest of the corrupted Nexus crystals that Leblanc could find. It was a tedious truce, but one necessary for the liberty of the world from the slowly, and insidiously, corrupting influence of the Institute.
Garen cleaved his way through minion after minion, hardly slowed. This is almost like fighting on the Rift, Garen thought. Even as Garen heard the screams of another member of the Dauntless Vanguard having fallen he realized that the tone of this fight was different. This was a fight to the death. Surveying the battlefield, Garen found himself beginning to worry, the minions were swarming from all sides, and the reinforcements that Katarina had promised were no where to be seen. Without Noxus, this battle would be a fool's errand. Even worse, Demacia would be handing the world over to the likes of Vessaria Kolminye and Jericho Swain on a silver platter with the destruction of their army.
Even as Garen began to fear the worst, for indeed he knew that enough minions would be able to harm even him! He heard a sound that brought a smile to his face.
"HERE COMES DRAAAAAAAAVEN!" A small cascaded of axes bit hard into the flank of the minions as Darius charged forward with a small host of Noxian troops, covered by Draven, Cassiopeia, and Urgot. Under normal circumstances Garen would have been aghast to see these four fighting side by side, but it did his heart good to know that these people, who were so often his enemies, were fighting beside him now.
With a shout of "Demacia!" Garen began his swinging, spinning dance into the heart of the minions. He couldn't afford to let the warriors of Noxus prove to be stronger than his Dauntless allies.
Lux and Leblanc moved deeper into the heart of the Institute of War, terrified that Lux's ability to bend the light around them would be dispelled by some enchant or another put in place by the Summoners who were hiding inside the massive, labyrinthine construction that was the Institute. They were looking for the chamber were the novice Summoners were bringing for wave after wave of minions, hoping to destroy the crystals that allowed them to bring forth their limitless army.
"This place is oddly familiar," Lux said, and before she knew what had happened the hall went dark.
Leblanc, standing beside her cursed their bad luck. A more powerful Summoner had found them and was attempting to enter their minds as they would have during a League Judgment. Before Lux could be carried away by the powerful illusions of the Summoners, Leblanc focused her own illusion magic into the area around them. Suddenly, there were warring images before Lux and Leblanc. It appeared that they were in the hallway, but they also saw Garen and Katarina fighting against each other, both obviously brutally injured. For a moment Lux almost let go of her enchantment of invisibility, the only thing from stopping the Summoners from knowing their exact location, before Leblanc put her hand on Lux's arm. Lux turned and saw Leblanc shaking her head before pointing out the surreal qualities of the scene. There was no way that Garen and Katarina could be there. They were both involved in the attack outside the building; the faint outlines of the walls ruining the scene even further. Lux nodded and continued forward, ignoring the Summoners insidious attempts to bend their minds.
Jarvan stood beside Jericho Swain, Talon, and Xin Zhao. His blood boiled at what he was about to do, but he knew that for all of his hatred of Swain; the Summoners were a far greater threat. Jarvan raised his spear high, flipping the switch that extended it to its full height. The core of the Demacian Army, several thousand troops overlooked one of the sides of the Institute of War. With a nod to Swain, the great war engines that had once been put to use in Ionia rolled forward with a new concoction created by none other than Singed. The heavy metal canisters flew forward from the catapults before crashing, and indeed exploding, against the walls of the Institute. However, the explosions weren't the real target. Within moments the corrosive liquid began to melt the marble that built the Institute of War, melting it away at a speed that was both profoundly effective and profoundly disturbing. Jarvan was happy to know that the liquid wouldn't damage the steel of his armor and weapons, nor the flesh of his body, but was terrified to think of what else Singed could be making at this very moment.
With a single forward wave of his lance Jarvan Lightshield the Fourth, Crown Prince of Demacia, led the charge into the heart of the Institute of War beside none other than Jericho Swain, Grand General of Noxus. Never had Jarvan or Swain thought that such an alliance would exist between them. Even as Swain transformed into the abomination of bird and man that Jarvan knew was his true form he felt the deep desire to drive his lance through Swain and end their war once and for all, as he knew that Swain must want to send his ravens to rip into his flesh.
But both refrained. The Summoners had to be stopped. They weren't trying to heal the world after the Rune Wars. They were trying to remake it in their image.
Side by side Garen and Darius fought on the steps leading into the main entry hall of the Institute of War. Sword and axe rending minions as if they were little more than illusions. Which, in truth, they were. As they approached the monolithic doors leading into the Institute they began to swing open, and Garen blanched at what he saw. Hundreds, maybe thousands, of the enormous "super" minions. They began to pour out onto the battlefield and Garen felt the tide beginning to turn against them. Darius nodded to Garen before gesturing into the heart of these larger monstrosities. The rank and file troops of Noxus were no match for such creatures, and even the Dauntless Vanguard would die two to one for each of these larger creatures they defeated. Their only hope was for men like Garen and Darius to someone stymie that overwhelming tide to give Draven, Cassiopeia, and Urgot time to do their work from range with the battalions of Noxian archers at the rear of the army.
So, Darius and Garen charged into the heart of these new-found enemies, blades swinging with abandon. Both men broke into the powerful, spinning swings that were their hallmarks, always moving perfectly to avoid injuring each other at the last possible moment to maximize the damage they were dealing to the colossal minions. But, even with all of their might, they were simply outmatched. If there had been half as many minions, they might have stood a true chance, but they were overwhelmed. Each minion that died had two, or even three, to replace them.
That was when the world stood still. The minions turned away from Garen and Darius and towards what seemed to be a beautifully built gargoyle. In truth, both Garen and Darius knew that it was Galio, using his magic to draw the minions towards him. Slipping into their minds and making him the beacon of the Summoners hate and fear. Then, a woman appeared beside Galio, and Garen, in spite of his loyalties to Demacia, felt his heart soar. Katarina had arrived. Within a moment she was a spinning storm of flying daggers. Always managing to land more than a dozen daggers into each minion before moving on to the next. Galio grunted, it was all he could do with how long he had held his taunt, and suddenly Katarina was beside Garen, smiling as Galio unleashed the energy of the attacks that the minions had been raining down on him back against them, shattering the center of the line of super minions. With a cheer for Demacia from Garen and Noxus from Darius, their armies continued to surge forward.
Vessaria Kolminye was scared. The Dauntless Vanguard and the Noxian Army had crushed their way through the front door, and the Demacian Army, led by some of the most dangerous leaders of Demacia and Noxus, had smashed their way into the Institute and were slaughtering the reinforcing lines of minions before they could even arrive to slow the march of the force that had taken the main entryway. What's more, three groups of novices were dead. Slaughtered with powerful magic by unseen foes, quite likely Lux and Leblanc. Somehow, in spite of all her power, she couldn't find her way into even one of their minds. If she could turn one of them, any one, against the others she could turn the tide. But, somehow, against everything Vessaria knew, they were immune.
And so, sitting with the other veteran Summoners, she was afraid.
Jarvan, Swain, Talon, and Xin Zhao stood before the doors they knew would teleport them to Summoner's Rift. As they prepared to move away, Lux and Leblanc appeared beside them.
"They are not here," Leblanc said, her svelte voice contrasting sharply with the splashes of blood on her face.
Swain responded, his voice eerily birdlike in his true form, "You have checked the entire building?"
"Yes, Grand General. There is only one place left for us to go," Lux said, gesturing to the door they knew would lead them to the Rift.
With a nod Jarvan led the way, charging through the door. As he appeared at the center of the Rift he was unsurprised to see the one champion who had refused to join them: Jax.
Garen stood at the head of the Demacian Army and Dauntless Vanguard, looking at Katarina and Darius at the head of the Noxian Army. They were at a standstill. All of the minions had been slain, and now it seemed that old hostilities might renew before the battle was even over. Without the head of Vessaria, this would all be for naught.
Darius tested the heft of his axe, seeming as if he would charge before Urgot surged to the front.
"Demacian," Urgot croaked through the hextech machines that gave him a voice, "tomorrow we might fight to kill each other. But, today, I would protect the Grand General, as you would protect your Prince. We both know where they have gone."
Garen nodded, and he ran with Urgot, Darius, Draven, Cassiopeia, Katarina, and Galio towards the doors that would lead to the Rift. Together they would win. That was the only choice now.
Jarvan dodged away again, even as Xin Zhao was sent flying backwards. Of all the infernal obstacles to overcome, Jax had to be the worst the Summoners could have put in their way. Armed with a beautifully crafted long spear, he was living up to his title of Grandmaster at Arms and beyond. Four against one and he hardly seemed to even be fighting. Each swing was perfect. Each movement was exactly what it needed to be for him to win, no more and no less.
Swain charged forward once more, his ravens charging at Jax. But, for all their numbers, for all the different angles of their attacks, Jax was never hit. Xin Zhao charged forward again, a shield of light flowing over him from Lux, whose spells just didn't seem to effect Jax at all. As if he was someone immune to the light. Leblanc threw spell after spell at him, but if they hurt him it didn't seem to matter. Jarvan slashed and stabbed, putting all of his speed and might behind each blow, sometimes even sundering the ground around him when he missed.
Yet, for all that, Jax would not be hit.
And then, suddenly, the fight was over. In a single, perfect, spin Jax sent Jarvan, Swain, Xin Zhao, and Talon (who had been attempting to align his daggers to assassinate Jax) were flying backwards. Lux and Leblanc could do nothing but watch as Jax stalked forward.
Vessaria was beaming. The greatest creation of the Summoners was doing exactly what they needed. When Reginald Ashram shattered his mind attempting a summoning that was beyond him they had thought he would be useless. But, with the right rune magic, they had taken him from being the most powerful summoner to being the most dangerous warrior in the warrior in Runeterra.
With Vessaria acting as the summoner, that was only amplifying his power. Making him stronger than any of the champions could hope to be without the symbiosis of a summoning. Even six against one, they were no match for Jax's power as she had enhanced it.
Through the doorway Darius and Garen saw what was happening to their beloved leaders, and they charged forward with everything they had. Darius leaped through the door, bringing his axe on high while Garen charged forward using his very will to charge magical energy into his sword. As Garen arrived he lunged at Jax, truly feinting. Jax knew this and didn't attempt to block, moving to stop the attack that was coming. A golden pillar of magic, in roughly the shape of Garen's sword, plunged down at Jax who rolled to the side and prepared to attack Garen again.
But Jax had made a mistake because Vessaria had made a mistake. Neither had noticed that Darius was falling from the heavens, his axe leading the way. At the last moment Jax moved his spear to block the powerful blow, only to have the weapon cleaved in two. As Jax tried to recover Katarina managed to sneak a dagger past his stumbling defenses to cut at his head. With a skill that no other man could rival, Jax attempted to dodge. His movements, so perfect, managed to divert the majority of the blows, but his hood and visor were blasted away.
Revealing, for all to see, the face of Reginald Ashram.
Leblanc smiled, jubilant at the sight. Channeling everything she had, Leblanc unleashed a cascade of images into Jax's mind; and therefore into the mind of Vessaria. The images were images of the life of Reginald Ashram, of his know successes and failures. Leblanc felt her energy failing, only to feel Lux place her hands on her shoulders and give her all of the magic she could, a move more motivated by desperation than any real skill with that ability.
Suddenly Jax staggered backwards as the leaders of Demacia and Noxus heard an otherworldly scream from behind Jax, towards the Nexus they knew would be present there.
As Jax staggered backwards he managed to mutter, "Who... where... what...?" Before collapsing to the ground, clearly overwhelmed.
Swain moved forward to stand beside Jarvan, reverting to his human form.
In the voice of an old man, tired from battle, Swain said, "You know what must be done. Leblanc has shattered them. Let my executioners finish this task and let us go our separate ways."
Jarvan nodded, as Draven swaggered forward.
"Always wanted to kill that witch," Draven declared, as he and Darius moved towards the few remaining Summoners, "she thought my mustache was stupid."
END.
TL;DR: I make better lore than Riot did while solving their problem. Not my best effort, but better than a retcon.