Hagrid and Sirius encountered Aragog after a lengthy enough walk through the woods. The conversation was... less than successful. Hagrid managed to convince Aragog to not, at least, aid Voldemort, as a favor to Hagrid in recompense for saving his life twice, but he could not, it seemed, risk sending his brood out to battle. He could not sire more, he argued, to replace any who died, which Hagrid and Sirius begrudgingly accepted as a solid argument. And so, they sought out Bane of the Centaurs.
This conversation went better. They promised the Centaurs some Wizards would enter the forest to protect them from Dementors, the ones they deemed the biggest threats, and then the Centaurs said they would give their aid, since they knew half-humans like themselves would suffer horribly under Voldemort's regime. Sirius thanked them profusely, but Bane simply answered that it was the least they could do, as the Wizards did, indeed, never interfere in their matters up till now. It was a small thing, but something the Centaurs would not overlook.
And so, Hagrid, Bane and Sirius returned to Hogwarts Castle... in time to witness a sea of black coming from the south. Sirius blanched. "Dementors." He whispered, before morphing into a dog and rushing up to the castle, Bane hot on his heels, while Hagrid went to get his umbrella, Fang, and whatever other aid he could scrounge up from the Forbidden Forest.
Minerva, who had seen the sea of black as well, turned to Dumbledore, "I hope your brother Aberforth is on his way," she commented dryly, trying to steel herself, "His Patronus is one of the greatest I've ever seen." Her words warmed Albus' heart. He always did appreciate good words about his brother; strong though the rift between them may be, he loves his family something fierce.
And so, together, he, McGonagall and Flitwick headed down the steps, where they met Sirius and Bane, the former of the two having morphed back to human. "Bane says he'll help, but he wants a wizard or two who can cast powerful Patronuses to protect them from the Dementors." Sirius panted, rushing inside to see who he could find. Dumbledore called back,
"What of the spiders?" His words echoed through the hall, but Sirius was already gone. Bane answered for him.
"Aragog and his kin are valuing their own skins, Dumbledore." His words were gruff, but respectful. "We Centaurs, at least, recognize the threat the Dark One poses. We will stand and fight, if only because when you fall, he will surely bare his fangs towards us, next." He explained, but was surprised to see Albus smile at him winningly.
"Thank you for the explanation, Bane. You will find, I think, that my brother casts an excellent Patronus, as Minerva so kindly pointed out to me a few minutes ago. He should be... ah, right there." The old wizard gestured ahead, to a similarly built, similarly bearded wizard who, at that very moment, launched three Patronuses into the cloud of Dementors, causing them to scatter and flee out of fear of the powerful 'happiness' the Patronuses represented. Bane grunted his approval, and went to explain to Aberforth what had been decided upon.
After a quick conversation, the duo hurried off towards the woods. If Bane had consented to let Aberforth ride him, they may have made it sooner, to prevent the deaths and Kisses that were already beginning to take place... but sadly, Bane's pride cost him those men and women. Something he will take to his grave.
From behind himself, Dumbledore heard Minerva call out, "Piertotum Locomotor!", and watched as scores of terracotta soldiers and suits of armor marched out to the boundaries as McGonagall issued commands, such as, "Man the boundaries! Protect us! Do your duty to our school!"
Dumbledore himself aided Filius in casting the protective enchantments around the school castle, creating a massive barrier that would vaporize any Death Eater who got in range. He also cast a spell that demolished all of the bridges, destroying them with a mere flick of the Elder Wand which he commanded.
Voldemort snarled as he watched this all unfold. His Dementors; repelled. His Giants... well, let's just say having only one path into the school made it difficult for them to wreak their havoc, especially when wizards as powerful as those within the School were guarding it with their lives. And his Death Eaters... they were useless. His Generals, such as Malfoy, Lestrange, Wormtail and Snape were wise enough to stand by his side and not kill themselves on this barrier, but the rest had charged recklessly, greatly reducing his numbers. Already the Battle of Hogwarts was going much in Dumbledore's favor, and then the Centaurs hopped into the fray.
Showers of arrows flew from the sky and rained down upon them, Voldemort and his Generals themselves only just putting their shields up in time, as they had never expected the Centaurs to rally under Dumbledore's Banner... but when he saw Aberforth Dumbledore at Bane's side, Voldemort knew, and his snarl became a roar of anger. He shot a Fiendfyre Serpent at the oncoming horde, and yet it was repelled by Aberforth's stunning strength, as he knocked it aside with his willpower combined with his magical might. One mustn't believe all the rumors they had once heard that Aberforth Dumbledore was nothing compared to his brother; they were still brothers, their strength WILL be at least somewhat comparable.
But then the battle began in earnest as Filius, Minerva and Albus descended onto the battlefield, launching powerful spells at the Death Eater Generals and Voldemort himself. They were outnumbered, but their magical might more than rivaled that of Voldemort and his Followers. McGonagall Transfigured a nearby stone into a tentacle which wrapped itself around Bellatrix Lestrange's waist, and flung her into the air. She shot a curse back, but Minerva deflected it as easily as if she were swatting a fly. Flitwick Charmed the hill they all stood upon to turn into a trampoline, causing Malfoy to lose his footing and begin bouncing up and down like a ball. Before Wormtail could curse anyone, however, Severus Snape held his wand to the traitorous Marauder's back.
Everyone froze, even Voldemort, who had been about to begin his second duel against Albus. "Abiciendi." Snape whispered, and Wormtail was shot off like a cannonball. "I am sorry, Headmaster, for my tardiness. But Severus Snape will join you in your efforts... to depose Lord Voldemort." He then turned his wand on Tom Riddle, who, now furious, unleashed an explosion of magical energy. Of course, Dumbledore expected this, and he cast a protective charm that spared all of his allies, including Snape, from the effects of the blast.
"YOU DARE--" Voldemort bellowed, launching a curse at Snape, before he suddenly was strangled by his own Robes. McGonagall had transfigured them into chains, and with a slow twist of her wand, they began strangling him further. He muttered the counterspell, and landed on his feet, even more furious. What he did not realize, however, was that his fury worked against him. He could not best so many powerful Wizards and Witches on his own, and his Lieutenants were either useless or traitorous, in the case of Severus Snape.
Snape rushed off to duel Wormtail, while McGonagall and Flitwick began dueling Lestrange and Malfoy, who had both recovered and were now flinging curses and hexes into the foray. It was just Dumbledore and Voldemort, now. And while Dumbledore would not survive if Voldemort's curse struck, only one of them truly knew that Voldemort himself would have his Horcruxes to fall back on, if all else failed. And so, the duels began, while inside the Castle, other powerful Death Eaters had begun hunting for Harry Lupin... unaware that there was not one, but three fathers protecting their children that night.
Remus Lupin stood at the entrance of the Hospital Wing, wand at the ready, Patronus by his side, glancing every which way to protect young Harry Lupin from his would be killers.
Elias Eaton, meanwhile, stood in front of Poppy Pomfrey's Office, wand in hand, where inside the three children slept under her watchful eye, including young Lancelius Eaton, Elias' own son.
Lastly, Percival Dumbledore stood at the windows of the Hospital Wing, waiting for flying enemies such as Dementors, wand out, as he braced himself to protect his daughter, Alexandra Dumbledore.
And just as they expected, the attack began. Antonin Dolohov, Rodolphus Lestrange, and Fenrir Greyback all three approached Remus Lupin, and together launched a trio of spells the Marauder barely blocked with an utterance of "Protego Maxima!" He skidded back into the room, and the three Death Eaters charged in, ready to begin their assault. But when they realized they were at equal - 3 on 3 - they seemed less excited...
Later, these Six Coming Duels would be called 'The Greatest Battles of the Century'.