Unlike the other races created by the Old Ones, they stood with the few who would remain to defend them, believing wholly in their great plan. They stood without hesitation or remorse against their former comrades in arms, using their great power to strike against them.
In the last battle, the great war was fought above the skies of their homeworld of Itza. There, their foes were the newly formed armies of the Lords and Angels of Order. Their leader, the indomitable Lord Kroak, used the vast powers he had received as a psyker in the lap of the Old Ones themselves. In a single vast strike, he sacrificed a million captured enemies to form a psychic defense that covered the entire planet. Upon its walls bashed the Militants of Khorne, the Lifebringers of Nurgle, and the Magi of Tzeentch, all to no avail. Nothing, from the mightiest Gargants of the Orks to the pinpoint strikes of the Eldar could penetrate it.
In the skies above, however, the Old Ones' battle raged and they were defeated when the Destroyer was sealed away through the sacrifice of Gork and Mork. Robbed of the source of much of their power, the shield weakened and the armies pressed against them increased.
In the space between, as the shield finally collapsed, the entire Slaan fleet assembled for a desperate final stand; they would hold off the enemy and give time to Lord Kroak to complete his great and final work.
Though the forces of Order were valiant and strong, the armies of the Slaan held for three days against their relentless assault, and when they were finally broken and troops landed on the surface, they were held back yet more by their expert guerrilla fighters, who took full advantage of the terrain of their jungle home.
At the last, this too did break, but it gave Lord Kroak the chance he needed to enact his final vengeance - a assault using powers of the warp that was the domain of no less than the gods themselves. With a thought, he sent out supernova levels of power, and formed black holes to devour entire enemy fleets. Whole ships were lost forever as they were swallowed by the winds of chaos.
At the last however, even this vast power was not to win the battle, as the cunning Eldar leader Eldrad had managed to maneuver himself into a position to strike, cutting down the lord of the lizardmen with a single blow to the back.
Even as he lay dying, however, Lord Kroak used the last of his power to scatter his people across the galaxy.
In the aftermath, his people recovered his body, and used all of their arcane arts to try to restore it to life. Such was his malevolence that not even death could stop him forever, and as the last few cells clung to life he ordered the construction of a vast throne, in which his body would one day be restored. In the mean time, his people would still serve the Old Ones.
Now the Slaan are returning, and Lord Kroak is regaining life. They seek the sealed Old Ones return, that they might serve them once again.