I'm Ashardalon. Really.
Ahem, as the others have said, he was first mentioned in Sunless Citadel, though the heroes propably only heard a rumor containing his name, or if they remove a goblin corpse obscuring his name.
(Forge of Fury and Speaker in Dreams don't mention him)
The Standing Stone is the next adventure mentioning his name, and also introduces the legend of Dydd, the druid who wounded the dragon's heart so he had to replace it. An image is carved into a hill in this adventure that some believe to be an image of Ashardalon.
Heart of Nightfang Spire deals with the undead cult of Ashardalon, and what became of his original heart.
(Deep Horizon is woefully Ashardalon-less)
The Lord of the Iron Fortress, as it happens, is one of the many children of Ashardalon, and he seems to search the dragon, as a note can be found that theorizes he is in the Abyss.
Bastion of Broken Souls finally has Ashardalon himself. He'll propably be killed, but the place where he is likely killed really is suspicious - he may yet return...
And thankfully, he has not been forgotten in the Draconomicon and Libris Mortis.
