ॐ अन्धं तमः प्रविशन्ति

12th Mantra of Isa-Upanishad. 

ॐ अन्धं तमः प्रविशन्ति येऽसम्भूतिमुपासते।
ततो भूय इव ते तमो य उ सम्भूत्याँ रताः॥१२॥

Into a blind darkness they enter who follow Unmanifested (Prakriti), they as if into greater darkness those who devote to the Manifested (Hiranyagarbha).