Idharae was ultimately destroyed in 852.M41 during an attack by the Invaders Space Marine Chapter, who with aid from the spectral Astartes of the Legion of the Damned reduced Idharae to a blasted, dead hulk drifting in the void.
The Craftworld fared so poorly against the Astartes because so many of its people had already been lost in an earlier conflict with Hive Fleet Naga.
The Naga Strikes
In 808.M41, a large splintered tendril of Hive Fleet Behemoth, later designated as Hive Fleet Naga, and known to the Aeldari as Shai'naid, the "Endless Winding Serpent," encroached upon a concentration of Aeldari Exodite Worlds in the Ybaric Cluster, which lay beyond the eastern fringe of the Vidar Sector. To their credit, Idharae, and its sister Craftworlds of Iyanden and Malan'tai, responded almost immediately, but even so, they were too slow.
By the time the first Asuryani fleets had engaged Hive Fleet Naga, Halathel, largest and most prosperous of the Exodite planets, was all but overrun by Tyranids. Even with aid of forces from Idharae and Malan'tai, the Exodites could not repel the invaders. Iyanden's forces, under the command of Admiral Draech, arrived at Halathel to discover its World Spirit destroyed and its defenders consumed.
Determined to exact vengeance for Halathel, the Iyanden fleet engaged the orbiting Hive Ships, but underestimated the menace of their foes. Draech's flagship, the Auspicious Illumination of Eternity, was destroyed early in the battle, and for a time, it seemed that the whole Aeldari fleet would be lost alongside it. Only when a young Prince named Yriel took command did the tide of battle turn.
Realising that the smaller Tyranid vessels could not function if the greater bio-ships were destroyed, Yriel converged his forces on these targets, but it was only when the Prince unleashed boarding parties to destroy the ships from within that he met with success. Though many Aeldari lives were lost and hundreds of Ghost Warriors destroyed, the bio-ships were finally slain.
With their passing, the smaller Tyranid vessels flew into an uncoordinated frenzy and were easy prey. Though the strength of Hive Fleet Naga had been greatly diminished, its threat was not yet ended. Over the following years, Yriel's forces joined with those of Idharae and Malan'tai to fight hundreds of engagements against the Tyranids, both in the cold dark of space and amidst the horror of partially-digested worlds.
Little by little, the Tyranids were scoured from the Ybaric Cluster, and victory was at last won -- though not without great cost. Idharae, which was never the most populous of Craftworlds to begin with, lost many of its warriors defending the Maiden World of Eth-aelas, and its halls were ever after empty and joyless places. Malan'tai suffered far worse and was destroyed when an aberrant form of Tyranid life devoured the Craftworld's Infinity Circuit and used the stolen power to slay the rest of Malan'tai's living Aeldari.
The Doom of Idharae
In the middle of the last century, in 852.M41, the Invaders Space Marine Chapter launched an assault against the Craftworld of Idharae, the Aeldari having been severely reduced in number following their actions in the war a half century earlier against the Tyranids of Hive Fleet Naga.
Despite sustaining heavy losses, the attack was a success. With help from the mysterious Legion of the Damned, the revenant Astartes blew out the great dome under which they fought, venting the Aeldari that were fighting out into the void. Such a bold action would have been unlikely to succeed were the Aeldari not so weakened by the war against the Naga, and it resulted in the Craftworld being reduced to a dead, drifting hulk of scorched and desiccated Wraithbone.
Four solar decades later, Craftworld Alaitoc launched a retaliatory strike against the upstart Astartes, venting their rage against the Invaders' Chapter homeworld of Ogrys. This time, the tables were turned, for now it was the Space Marines that were under-strength, their numbers still returning to their full complement following the losses they had sustained during the destruction of Craftworld Idharae. The Chapter was overwhelmed, their fortress-monastery destroyed and Ogrys reduced to ruins. Only 300 Battle-Brothers survived the battle, after which the Invaders were forced to take to their remaining starships as a fleet-based Chapter, capable only of actions in support of larger Imperial formations.
- The Attack of Shai'naid (808-812.M41) - Hive Fleet Naga encroaches upon the Ybaric Cluster, which lay beyond the Eastern Fringe of the Vidar Sector. Though the Tyranid swarm touched little on Mankind's worlds in the region, countless alien domains suffered from its embrace. Two Aeldari Maiden Worlds and seven Exodite Worlds would be lost to the Hive Fleet's rampage. Fleets from Idharae, Malan'tai and Iyanden converged upon Naga and vanquished it in a series of bloody strikes that only the Aeldari could have performed with so few casualties.
- The Doom of Idharae (852.M41) - The revenant Astartes known as the Legion of the Damned come to the aid of the Invaders Space Marine Chapter as they wage a devastating attack upon Craftworld Idharae. When their flaming bolts do little harm to the raging Avatar of Khaine at the heart of the battle, the Legion instead blows out the great dome under which they fight. They vanish once more as the Eldar defenders and the single surviving member of the Invaders force left fighting there are blasted out into the void. Idharae is left as a ruined and desolate hulk.
- Vengeance for Idharae (895.M41) - Four solar decades later, the Aeldari enact bitter vengeance upon the Invaders Chapter when the forces of Craftworld Alaitoc launched a retaliatory attack upon their Chapter homeworld of Ogrys. With the Invaders still under strength following their assault on Idharae they are quickly overcome. Though three companies escape the disaster, the fortress-monastery is lost. The Invaders are thereafter a spacebound, fleet-based Chapter.
- Codex: Legion of the Damned (6th Edition), pg. 18
- Codex: Space Marines (5th Edition), pg. 49
- Codex: Tyranids (5th Edition), pp. 16-17
- Iyanden - A Codex: Eldar Supplement (6th Edition), pg. 12