Endless Darkness! (I)
The Primarchs focused on the many High Worlds containing billions of life forms as they traveled toward the White Sun. None of them dared to release their consciousnesses, but their eyes were more than powerful enough to see the Neo-Demons despite the distance.
One of those High Worlds had a shocking gravity, but the Neo-Demons that normally inhabited it had no problem with it. The reason for that was what drew the Titan Primarch's attention.
All the Neo-Demons that had taken that High World as their home had some variant of Titan Bloodline, making their constitution incredibly sturdy since their birth.
Usually, the offspring of individuals with transplanted bloodline would have a very low purity of said bloodline. Still, all the children that the Titan Primarch saw were as strong as natural-born Titans.