Thank you for clarifying the timeline! I mixed up when they killed off a bunch of their kin with when they started siphoning the demon.Anyhow, the Shen'dralar timeline is as follows:
- -10,000: The Sundering. The Shen'dralar remain isolated and secluded in Eldre'Thalas. Since they are becoming lethargic, they summon the demon Immol'thar and start siphoning its power. [World of Warcraft: Chronicle Volume 1, pg. 133]
- -1,200: The Slaughter. Prince Tortheldrin and his followers kill most of their Shen'dralar elven kin in order to hoard the fading demon's power for themselves. Most of the city is abandoned, and other creatures start moving in. [World of Warcraft: Chronicle Volume 1, pg. 139]
- 25: Dire Maul. The ruins of Eldre'Thalas are raided and the Shen'dralar discovered feeding on Immol'thar. The majority of elves, including the prince, is killed by the Horde, while the demon is banished into the Twisting Nether. The surviving Shen'dralar flee the ruined city and wander in the woods trying to wean themselves off demonic power addiction. [World of Warcraft: Chronicle Volume 3, pg. 120]
- 28: The Cataclysm. The Highborne Shen'dralar sorcerers from Dire Maul eventually settled in Darnassus.
This is supposedly why, contrary to the Nightborne, the Shen'dralar did not change appearance much if at all, and Brann Bronzebeard reports just that thay have a 'pastier complexion'. [World of Warcraft: The Magazine Volume II Issue I] (pic from Wowpedia)