We finally concluded the last battle, which was appropriately dramatic (I hope). The PCs saw starlight filtering in from the exit to the dwarven ruins they'd been navigating, and heard the chaotic sounds of hundreds of voices. The pixie buffed and flew up to scout, and saw that there was an army of gnolls camped just outside, along with the cleric they'd defeated earlier in the game, his horde of undead, 11 black unicorns (vampire unicorns) with vampire spawn riders, and finally the BBEG himself: a bald, purple-robed, flying vampire wizard. The BBEG had See Invisible up (he's a divination specialist, and his minions had tangled with the PCs numerous times, including the vrock very recently, so he was well informed of who they were and when they'd show up). So he saw her and tried to hit her with a Glitterdust. They rolled initiative checks, and she won by 1 point, so I said she managed to get out of the way before it "landed". She flew back into the cavern, and they began buffing and summoning like they never had before. I offered several tips about useful defensive spells, and they used their "ace" and called the dragon that they'd reincarnated earlier. It had originally been a black, but was reincarnated as a copper (mature adult). It came along with its two half-(black)dragon dire wolf children. She also began buffing, and before long they all had Death Ward, a spell that let them bypass DR/silver, and Resist Energy: Fire. The druid and his companion also had Luminous Armor, and he summoned a quartet of greenbound hippogriffs. The hippogriffs laid down a continuous supply of Wall of Thorns in the tunnel exit, with the dragon spewing acid into it as frequently as possible. They massacred a huge number of bugbears trying to rush through the bottleneck.
Eventually half the black unicorns and their riders came down the tunnel with bugbear reinforcements. The dragon breathed and wiped out the spawn and bugbears, but the unicorns kept coming until a lucky turning roll by the lone remaining good unicorn (who had been buffed by me to a celestial charger
) turned them all...
Then they hatched a plan: the druid summoned an earth elemental, and turned into a small bird. The earth elemental then picked up he and the pixie and carried them to the surface through the rocky mountain above them. They sent the elemental back, and the pixie hid and slowly crept to the top of the "tallest mountain" with the Beacon of Hope (MacGuffin) while the druid flew up (as a rhino) and went mano-a-mano with the wizard and his called vrock (who had been healed back to full since they last encountered him). With a stoneskin effect from Heart of Earth, as well as his other buffs, he was holding his own against them for several rounds. He was wearing a Hathran Mask of True Seeing he'd borrowed from the pixie to bypass their Mirror Images, and his buffs also let him bypass their damage reduction. The wizard's 30 AC was a bit tough, though. He summoned two giant crocodiles, imbued with Celestial Aspect for wings, and together they managed to land a few good blows on the wizard, and even grapple him for a round (until he activated his Heart of Water for Freedom of Movement).
Finally, the weak sun crested the horizon, and the pixie finished the battle with two dramatic flourishes. She used her Horn of the Androsphinx to cause a cave-in and crush most of the army, and trap the rest of it within the cavern. She also held up the MacGuffin (Beacon) so that it focused the sun's weak light into a beam that first destroyed the BBEG, then transformed the "black unicorns" back into their normal selves, banishing the darkness and saving the world.