Travel to the Lifeblood Pillar and look for clues to its destruction.
The titans shaped entire life forms here, fostering their unfettered growth.
The five crystal pillars that surround the basin emanate pure lifeforce. The Scourge should've never been able to penetrate the basin, but one of the pillars was destroyed.
Go to the Lifeblood Pillar and see if you can find out what happened. No undead would've been able to approach the pillar, so keep your eyes out for living intruders. You'll find what remains of the pillar directly south of here.