Durthu lied to the elves about where the Ever Daughter was kept. He told them that she was kept in a sacred, protected location within Averlorn:
"She is Yvraine," said Oakheart*, his voice a melodic noise like the sighing of a light wind through branches. "Though Astarielle remained to protect Avelorn against the daemons, she bid us to take her children to safety. To the Gaen Vale** I carried them, where no other elf has trod. There my kin and I fought the daemons and kept Yvraine and Morelion safe those many years.
* Durthu assumed the identity of Oakheart while pretending to be an Averlornean Treeman, IIRC.
When strife once again began to assail Ulthuan in the insidious guise of the Cult of Pleasure, may elves chose to return to Ulthuan, but there were those who believed that the Phoenix King and his rule had little to do with their lives, and many of these elves moved themselves further inland.
Athel Loren
it was these elves who first encountered the strange and frightening forest that spread from the foothills of the Grey Mountains and the Vaults. The forest was filled with all manner of life, and the Elves constantly felt eyes upon them. At night they saw strange lights dancoing in the darkness, and huge shapes lingered on the edge of the glades. Intrigued by what they saw, the Elves attempted to push further into the dark forest, but they found themselves thwarted at every turn.
Three times did the Elves send expeditions into the forest, each numbering nearly a thousand warriors. Of the first expedition, seven hundred Elves came out of the wood scant hours after they departed, with tales of many fruitless days spent travelling along shifting glades and pathways that curved back on themselves. The second expedition vanished without a trace. Of the third foray, only a single elf returns, her face ashen and drawn, and her body lacerated and bloody. She died a day later, driven witless by her dreams. It was clear that the place perceived the Elves as a threat, and was actively resisting their attempts to dwell with its depths The Elves realised that this magic forest was sentient - like one giant all-encompassing living creatures, and those that it saw as a threat, it would resist.
The elves named the forest Athel Loren. They retreated to the outskirts and placed great magical Waystones about the boundaries.
...
Description of War of the Beard
...
The Birth of the Wood Elves
Following their victory, the Elves began to dwell deeper within the borders of Athel Loren, now fearing reprisals from the dwarves more than they did the capricious nature of the forest.
...
Each group pushed deeper into the forest, deeper than any had gone before. Strangely it seemed that the forest did not resist them, indeed it seemed that Athel Loren now chose to open up many of its secrets to the kinbands.
It was only after the War of the Beard that Athel Loren allowed the elves into its interior.
