After the Lich King conquered Azeroth, it was only a matter of time before the Burning Legion renewed their invasion, eager to collect their wayward servant and press him and his undead Scourge into their fold.
Meurryel the Ashmother would be the one who made a pivotal discovery about the demons.
As a Forsaken, she defied the Scourge, plying her demonic arts of shadow and flame (especially flame) against them. Her last stand culminated in a wreathe of flame so great that it burned the ghouls, and herself, to cinders. Her final act would be in vain, as her ashen corpse was reclaimed by the Scourge to serve once again.
The Scourge’s investment would be returned tenfold when the Meurryel, now the Ashmother, learned that demons regenerate in the Nether when slain. She quickly realized that the best way to prevent demons from returning after death was to ensure they never died in the first place. By damning the Legion’s servants to the curse of undeath, she circumvented their greatest strength.
I’ll throw the Design Insight post on this piece a little later, stagger things a little bit.