I ran a fun dragon encounter recently for my party.
The best part was that the dragon was "dead" when they found it.
The party was exploring a dungeon complex that they knew was the lair of a dragon who worshipped Tiamat. They were suitably worried.
They had also been told that this dragon took maidens captive on a regular basis for torture, sacrifice, and general snacking.
The party fought their way through the dungeon, and finally reached the beast's lair.
There they found nothing but an enormous mound of treasure - a large dragon hoarde, but no sign of the dragon.
Further searching found a captive, chained to a wall. After tending her wounds, the party questioned the hysterical maiden who told them that the dragon had left after a fight with a nearby giant.
Thgis confirmed what the party had learned earlier when they met the giant. They knew there was bad blood between the two, and that they had clashed recently.
Being good-hearted noble heroes, the PC's decided that they needed to get this poor woman back to safety. However, they were still on a quest for an item rumored to be in the dragon's hoarde, and after all, there was a huge treasure mound to loot ... so they
SPLIT THE PARTY!
The Ranger/Horizon Walker and the Rogue/Shadowdancer were to escort the woman back to the party's secure base-camp, where their cohorts and followers could see that she was returned to her people. The Paladin and Cleric/Wizard/Mystic Theurge remained behind to sort/loot the treasure.
The escort team made it abnout halfway out of the dungeon when the captive showed her true colors. With a clever use of the Magic Jar spell, the dragon had occupied the body of a female warrior, who proceeded to beat the holy heck out of her "escorts." Once they were disabled, (one KO'd, the other paralyzed) she dumped the warrior body, and returned to her original draconic body - which was hidden in a recessed compartment beneath her hoarde of treasure.
Bursting from the mound of loot, the dragon lit into the Paladin and his spell-casting companion with everything she had. The fight was glorious. Without the help from their teammates, and worried that their friends might be dying somewhere, the Paladin and the spellcaster fought like devil himself. They used irreplaceable magic items, exhausted spell lists, and after a brutal conflict, managed to slay the beast.
With a handful of hit points left they rushed to find and rescue their comrades, and only then finally complete their quest
My group still talks about that dragon fight because of the sneaky tactics the dragon used. They were used to arrogant dragons, who spouted their invincibility Smaug-like, cocksure of their draconic superiority. Giving them a dragon that knew it could be beaten in a straight up fight, and chose to behave in a sly, crafty manner really made the fight come alive for everyone.
Still hoping for Elements of Magic: Mythic Earth