Cultural event
"The loss of Aeren is something that all Elves felt keenly, and they were apparently determined to never lose such a soul as Aeren's again. After years of study and research by Elven sages the Undying Court of Aerenal became a reality, with their old king, Ivaren, becoming the first of the positive energy undead. I Think once the wars with dragons broke out those same sages wished they had put more thought into how to fight the scaled monsters"
The History of the Elves By Professor Zeldyr of Morgrave University