Nerath was destroyed by hordes led by the mysterious "Ruler of Ruin". (Dr.393. p.12)
CY-190: In Nerath’s final days, King Elidyr recalled the garrisons from the frontiers, urging them to surrender their posts to shore up the cities against the gnoll hordes. Many returned to their native lands, but some stayed behind to shield the flagging kingdom against the other arrayed enemies as they had since the kingdom first spread beyond its first cities. (DR386, p.12)
CY-190: King Elidyr fell in battle during the gnoll wars. The battle broke the Nerath empire and led to its downfall. (HoS, P.6)
CY-190: King Elidyr died, along with his heirs and trusted nobles, a “few decades” before the final disintegration of Nerath. Other sources claimed that the fall of Nerath involved orcs, goblins, demons, and perhaps a secret curse.