Elledariah

At one time, the elves had a small stronghold in northern Ustalav. It was established to guard the Lichgate, the last remaining elf gate that linked to the demon-corrupted Tanglebriar.
The garrison's purpose was intended to be purely defensive, but its commander, an elf called Elledariah, saw an opportunity to establish a base within Tanglebriar and take the war to the demons. Unfortunately, this plan enabled a succubus to infiltrate the elven forces, and ultimately to influence them such that they became introverted and paranoid.
Under the demon's influence, the garrison stopped keeping in contact with the main elven forces in Kyonin – resulting in a patrol being sent to investigate the situation. The corrupted elves mistook the patrol for demons in disguise, and ambushed them. Only Elledariah and the disguised succubus survived the fight – at which point the succubus revealed her true nature to Elledariah before returning to Tanglebriar in triumph.
Overcome by remorse and despair, Elledariah took her own life – only to rise as a banshee.
Today, the elves have quarantined the region around Lichgate, but have not yet got around to trying to lay the banshee to rest.