Emperor's Children - Gene-Seed Crisis and the Blight: One great disaster made the greatest mark upon the early solar decades of the IIIrd Legion's existence. This was the catastrophic loss of nearly the entire gene-seed stock of the Emperor's Children. Coming within a solar year of their great moment of triumph fighting at the Emperor's side at Proxima, it was to prove a turning point that was to forever alter the IIIrd Legion's future.As the Emperor's Wars of Unity broke the bounds of Terra, the pacification of the Selenar gene-cults of Luna and the Treaty of Mars allowed the Imperium to produce and equip new Space Marines at an unprecedented rate, and the Legions began to expand to meet the demands of the vast new war across the stars.As the gene-forges of Luna began to implant recruits for all the Space Marine Legions, a portion of the IIIrd Legion's gene-seed reserve was dispatched to Luna for establishment there. What happened next is not clear.Some claim that elements of the Selenite cults still resistant to the rule of the Imperium and the Imperial Truth hijacked a defence laser and destroyed the ship carrying the IIIrd Legion's gene-seed, while conflicting accounts recount that the ship lost control and crashed as it was attempting to dock, whilst others claim that it simply vanished.The loss of the IIIrd Legion's gene-seed reserve was a severe blow to the Legion's development, but it would not have endangered the Legion's survival if a second calamity had not occurred in quick succession.Like all of the Legiones Astartes, the IIIrd Legion recovered the Progenoid Glands from those of its warriors who fell in battle. From these organs a fresh set of gene-seed implant organs could be grown and a new Astartes created to replace the fallen. This system was, however, far from perfect.The nature of battle, and the manner in which Legionaries died, did not always allow for such recovery. To ensure that there were always organs ready to implant into new aspirants, a reserve of gene-seed for every Legion was kept safe on Terra. From this emergency reserve it should have been possible to keep the IIIrd Legion supplied with new warriors, and even with the loss of the gene-seed reserve sent to Luna the Legion would have endured and in time grown; its survival should have been certain. But in a single night that hope was obliterated.It was discovered that a fast-acting viral blight had suddenly infected several of the gene-seed vaults on Terra, its cause and origin unknown. The Genetors of the Mechanicum tasked with overseeing the gene-stocks feverishly sought to hold it in check as its progress threatened to wipe out in a matter of solar hours what had taken a standard century to build, but the doubtlessly artificial, and many surmised xenos, infection defied treatment, and it was only the intervention of the Emperor's own peerless bioengineering genius that was to purge the taint.While many Legions suffered losses from this biological attack from an unknown quarter, the Blight was found to have destroyed the remaining reserve of the IIIrd Legion's gene-seed stock in its entirety. From that moment the IIIrd Legion began to die.While other Legions grew in size and glory as the Great Crusade gathered pace, the IIIrd Legion withered. The only way it could replace losses was from the Progenoid Glands of the dead. Without the Legion's primarch to serve as a template for rapid replication, the Emperor and His gene-wrights of the Biotechnical Division could only rebuild the IIIrd Legion's gene-seed reserves with painful slowness.As the process of rebuilding crept forward, the IIIrd Legion's strength dwindled with every battle. It became clear that the IIIrd Legion would have fallen far below effective strength long before their gene-seed reserves were rebuilt. The doom of the Legion was inevitable -- and then the primarch Fulgrim was re-discovered, and everything changed.