The Calamity

The Calamity is the most common name used to refer to whatever cataclysmic event caused the great civilizations of the world to disappear. It's suspected to have been some great war or natural disaster, and its effects are felt all over the world. None living recall a time before the Calamity and the exact timeline of when it happened is not clear - even very long-lived races don't recall the details of when or what it was.
A few things are generally accepted as known about the Calamity:

• The world was more populated and more organized before it happened.
• Many of the scavenged items recovered by people in the present were created before the Calamity, and the knowledge to recreate the items or their effects is lost.
• Some areas have unusual weather, ecosystems or topography as a result of whatever caused the Calamity. Many of these areas are considered unpredictable and dangerous.
• Some records (usually stories or legends) persist from the time before and refer to things people do not see in their day-to-day lives (warforged, dragons, extraplanar creatures and so on).