Harrowing

The Harrowing is the elven term for a time when one of their regular conflicts with the Il'yana turned unexpectedly bloody and vicious, devastating a couple of the southern provinces of the elven empire.
An il'yana commander, careless of the consequences, loosed a swarm of vrocks to pillage and ravage as they pleased. At the same time, seeing an opportunity, the newly conquered gnolls rebelled, following in the wake of the vrocks to scavenge and steal what they could before fleeing into the wilds.
What took the elves by surprise was how the clustered power of the vrocks (and possibly a few alkilith) created rips in reality, loosing more and more fiends until finally several of the Titans had to intervene. To prevent this occurring again, the gods created the Veil.