The Djen was a large group of djinn and their slaves and servants led by the powerful djinni Calim. They came to Faerûn from an unknown place around -7800 DR and settled the areas immediately surrounding present-day Calimport. This event was known as "The Great Arrival" and within mere months, they had built a large city, whose spires quickly rose above the trees.
They then proceeded to clear and seize all of the lands south and west of the Marching Mountains but unfortunately, after only 10 years, their city fell to a flight of dragons. Although with the genie's magic, the city was rebuilt within a year, the attack caused Calim to lead the Djen in a campaign against the dragons of the Marching Mountains. Within 100 years, all of the dragons in that mountain range were dead.
The Djen empire lasted nearly two millennia with Calim at its head but its existence was fraught with war, first with marids, then with efreet led by Memnon. The battles with the efreet were particularly destructive.
Eventually, tired of the senseless destruction, Elven High mages disembodied Calim and Memnon, causing the creation of the Calim Desert. Afterward The human and halfling slaves and servants allied with the dwarves of Shanatar to drive the rest of the genies out of the area, creating the nation of Coramshan.