The Day of Mourning is an event where the nation of Cyre was destroyed in a magical holocaust of unknown origins on the 20th of Olarune, 994 YK. This event marked the beginning of the end of the Last War. Over a million Cyrans perished, including Cyre's Queen, Dannel ir'Wyrnarn along with military personnel from Breland, Thrane, and Karrnath within the nation's borders.
The cause and origin of the Mourning is unknown. There are conflicting eyewitness accounts that include a blinding light on the Saerun Road, or mists originating from the capitol of Metrol . However, by the end of the day the mists had advanced to form a wall around what once was Cyre. The land was subsequently named the Mournland.
Day of Mourning
In the days before the Day of Mourning occurred what is now known as the Field of Ruins in southwestern Cyre, the combined forces of the Brelish military, led by Princess Borann, and Thrane armies, led by Bishop-Militant Grodan, a close friend and advisor of High Cardinal Krozen, joined by pact, were attacking a much smaller Cyran force. The Cyrans had just been reinforced, after having cleverly used tireless warforged units for days to convince their opponents that they possessed greater numbers, but were still outnumbered 3-1.
However bad things might have looked for the Cyrans, no one could have imagined the devastation in store. And the invading armies were not spared. The skies are said to have burned so brightly that soldiers at Angwar Keep in Thrane were blinded . In the end, virtually everyone within the borders of what is now the Mournland perished. The only exceptions were those who could escape or survive via magical means, and those who were on the borders or outside of Cyre.
When news broke of the events of the Day of Mourning, offensive operations of the other nations ceased and peace talks began, culminating in the Treaty of Thronehold and bringing an end to the Last War in 996 YK.
In other media
The Day of Mourning is an iconic and cataclysmic event in Eberron's history, and so is touched on in many Eberron adventures, novels, and games. While these are not considered part of Eberron's canon, they may provide inspiration for your own games.
In the idle game, Champions of the Forgotten Realms, the Day of Mourning caused an enormous subterranean prison complex—built by House Cannith to contain alien and extraplanar threats for all time—to fall and its jailer, the warforged Warden became corrupted by the very things that the construct was created to imprison. 
- ↑ Eberron: Rising from the Last War, p. 109. Keith Baker, Jeremy Crawford, and James Wyatt (2019). Wizards of the Coast. ISBN 0786966890.
- ↑ 2.0 2.1 Eberron: Rising from the Last War, p. 14. Keith Baker, Jeremy Crawford, and James Wyatt (2019). Wizards of the Coast. ISBN 0786966890.
- ↑ 3.0 3.1 The Forge of War. James Wyatt, Wolfgang Baur, Ari Marmell (2007). Wizards of the Coast. ISBN 0-7869-4153-7.
- ↑ Eberron: Rising from the Last War, p. 154. Keith Baker, Jeremy Crawford, and James Wyatt (2019). Wizards of the Coast. ISBN 0786966890.
- ↑ Sharn: City of Towers, p. 32. Keith Baker & James Wyatt (2004). Wizards of the Coast. ISBN 0-7869-3434-4.
- ↑ Eberron Campaign Guide, p. 272. James Wyatt and Keith Baker (2009). Wizards of the Coast. ISBN 0-7869-5099-4.
- ↑ "Idle Champion Spotlight: Warden". Dylan Wilks (December 3rd, 2018). Codename Entertainment.
- ↑ Codename Entertainment (September 2017). Idle Champions of the Forgotten Realms. Codename Entertainment