Dark Sun adventures need more epic level action, so as a grand finale to a high level epic campaign why not pit players against the Athasian end-game?: Rajaat.
He stood twice as tall as any giant, with a crown of lightning crackling around his head. A constant crash of thunder belched forth from his fang-toothed mouth, and whenever he exhaled, billowing blue fog shot from his gaping nostrils and dissolved in a torrent of rain. His entire body was swaddled in roiling clouds the color of turquoise, and great torrents of salty water poured from the claws at the end of his gangling arms. Even his shadow was part of the tempest, causing the water to chum and froth wherever it fell.