Space marine candidates are first chosen when they are children. Custodies candidates are picked out when they are still babies (this allows them to be grown and genetically changed before they grow
The newer the lore, the stronger the Custodians are. From getting killed by naked World Eaters to soloing entire armies. Their purity is still unquestionable as Drachnyen the echo of humanity's first murder that fought the Emperor as a caveman stated that entire stars would die before any might be able to fall. Same in the heart of Chaos on the Vengeful Spirit where Horus uses the full power of Chaos to overpower Custodes and force them to fight their liege who were still spiritually pure. Custodian purity is proven again when Angron managed to corrupt an entire Imperial task force to Khorne including Sisters of Battle and Black Templar Astartes except Custodians, Sisters of Silence and Grey Knights.
Space marine candidates are first chosen when they are children. Custodies candidates are picked out when they are still babies (this allows them to be grown and genetically changed before they grow
The newer the lore, the stronger the Custodians are. From getting killed by naked World Eaters to soloing entire armies. Their purity is still unquestionable as Drachnyen the echo of humanity's first murder that fought the Emperor as a caveman stated that entire stars would die before any might be able to fall.
Same in the heart of Chaos on the Vengeful Spirit where Horus uses the full power of Chaos to overpower Custodes and force them to fight their liege who were still spiritually pure. Custodian purity is proven again when Angron managed to corrupt an entire Imperial task force to Khorne including Sisters of Battle and Black Templar Astartes except Custodians, Sisters of Silence and Grey Knights.