Lord Zardoz said:
I am of an open mind on this, as long as the story is better then "this one time, we fought a half dragon, and it was kewl". If you can top that go ahead.
END COMMUNICATION
Alright, then. In this one campaign that I ran (the same one as with the White Dragon), the bigbad was a half-orc, half-Black Dragon named Serecc the Conquerer. Unlike most half-Dragons, his mother (an Orc sorceress of legendary power) actually forced herself upon a polymorphed dragon (normally, of course, it would be the other way around). Anyway, Serecc was first known as a solid entity after a rather large quest wherein the PCs sought out a few specific relics to complete a ritual which would undo a field of temporal storms that a mummy named Azar Keth had wrought. Following their completion of the ritual, a detachment of Serecc's army (under the command of one of his four elite generals, a Wild Elf Psychic Warrior named Ahalya) moved in and occupied the city. The party dealt with them rather handily, and then set fire to the town. As they fled, they heard Serecc (who arrived at the town moments after they escaped into the brush) release a soul-shattering roar. They would later find out that this was because Ahalya was his adopted daughter.
The party's home base was on a demiplane, connected to the Material Plane by a few doorways that acted as Gates. About 1/3 of the way through the campaign, this demiplane (a Psionics academy, actually) was invaded by Serecc. Their headmaster was slaughtered before their eyes (decapitated with a single swing of Serecc's claws, thanks to his 32 STR), because he would not reveal the location of the Artifact which held the demiplane together: the dread Annulus. He basically tore his way through the academy searching for it, and, eventually, found it. As he claimed it, the demiplane began to destabilize, and everyone decided to cheese it. Serecc was not to keen on that happening, and ended up killing the party's cleric. The cleric, Squall, had almost made it to one of the Gates, when Serecc just hauled off and threw his spear (the Guurgal, from Weapons of Legacy) right through Squall's head. His brain was kind of... missing. A majority of the student body was melted by Serecc's breath weapon over the next few moments, but the party managed to escape, and took Squall's body with them.
Serecc would have been the final boss, but... I kind of achieved a TPK in the boss fight immediately before Serecc. It was a pair of Elf Sorceresses, one Sorc/Rog/Asn, and the other Sorc/Ftr/Eld, and the fight lasted over 3 hours. That was amusing, but not strictly relevant. In the end, Serecc spread a final darkness over the land, and all life was extinguished. He escaped this fate by means of Asmodeus, Lord of the Ninth Circle of Hell, who promoted Serecc to Lord of the Third, and removed him from the Material Plane just as the wave of death started to spread.
The players were kind of mad...