Finally managed to get my copy of the Iyanden supplement off my mother who was too lazy to buy her own copy so 'borrowed mine'. I'm really, really impressed with it, the background is excellent, well thought out and adds a huge amount of depth to Iyanden and the eldar in general. Some highlights:
-Iyanden pioneered the use of soulstones and using the Infinity Circuit to store souls.
- They had an alliance with Biel-Tan to cleanse the galaxy of Chaos with Biel Tan focusing on the Western rim and Iyanden on the eastern. They ended up losing contact after Biel-Tan started focusing on fighting anyone, not just Chaos.
-The history of Iyanden post fall is written as a metaphor for the Fall of the eldar, it's very effective. Basically Iyanden was so successful in its campaigns in the Eastern rim that they became complacent and arrogant and ultimately ignored Eldrad Ulthrans warning about the 'nids
-Idharae and Malan'tai both allied with Iyanden as part of their vision for a new Eldar Empire. Idharae was shown to be a small but great craftworld that was severely depleted of manpower fighting against Hive Fleet Naga which goes some way to explaining how a single Space Marine chapter was able to destroy it
-The story of Yriels exile is detailed, I thought it was rather good and fit well with the overall themes of Iyanden. Long story short his arrogance let a Chaos attack do some significant damage to the Craftworld itseld (wiping out Iyanna Arienals family too).
-The avatar still gets ownt by carnifices but it isn't quite so stupid now as it comes very late in the final battle and the avatar had been fighting a lot already so he didn't just wake up and get killed like an idiot.
-Yriel sees all sorts of visions while chatting with a Shadowseer as he is about to take up his cursed spear, including the Eldar gods being reborn.
-Iyanna Arienal is hunting down relics of Morag-Hai which she believes will allow the eldar to awaken Ynnead early, perhaps with the sacrifice of Iyandens living population.
-There is a sacred Flame of Asuryan on the Craftworld which is supposed to be the last fragment of Asuryans power that wasn't et by Slaanesh.
Heaps more I'm forgetting, I'll read it again and post more. If you are at all interested in eldar I really recommend getting this book.