The Seeds of Sehan campaign arc was a set of three linked adventures which appeared in the pages of Dungeon magazine:
Vile Addiction (Dungeon #145)
A horrific drug has seized the population of the strange city of Exag, yet confronting its source only reveals the true extent of a dire new threat. Part one of the Seeds of Sehan campaign arc, this is a D&D adventure for 8th-level characters.
Spawn of Sehan (Dungeon #146)
Within an ancient necropolis a pool of alien slime grasps the minds and bodies of the people of Exag with wispy tendrils of madness. Those transformed by its power invariably seek the crypt, but what fell power draws them there? Part two of the Seeds of Sehan campaign arc, this is a D&D adventure for 9th-level characters.
Dread Pagoda of the Inscrutable Ones (Dungeon #147)
Atop a distant mountain peak, a monstrous entity plots dark deeds. The final chapter of the Seeds of Sehan campaign arc, this is a D&D adventure for 10th-level characters.
Dungeon #145 also featured a supporting 'backdrop' article on the city of Exag.
All three adventures, plus the backdrop article, were credited to Tom Ganz, Stefan Happ, Stephen S. Greer, B. Matthew Conklin III, and Ashavan Doyon.