Īśāvāsyopaniṣat 03

असुर्या नाम ते लोका अन्धेन तमसाऽवृताः। तान् ते प्रेत्याभिगच्छन्ति ये के चात्महनो जनाः ॥३॥

Those worlds named 'Asura', i.e. demoniacal, are enveloped in the darkness of ignorance. Those offenders of 'ātman', i.e. offenders of their core essence, attain such darkness after death.

Īśā.Bhāshya 03

'Asurya' - having the nature to delight in that which is contrary to good. Indeed, they worship the non existent, promote nihilism, and are killers of the 'ātman', the essence. By stating 'ye ke' , i.e. 'those who', the governing principle that offenders of 'ātman' attain dark region is conveyed. The 'ātman' mentioned is nothing but Lord Hari is substantiated by quoting testimonials from Bhagavata and Vāmana Purānas.

सुष्ठु रमणविरुद्धत्वात् असुराणां प्राप्यत्वाच्च असुर्याः।

Those who delight in that which is contrary to good are demons. Having such a nature is 'asuryā', i.e. demoniacal.

"न च रमन्त्यहो असदुपासनया आत्महन"

"Indeed, they are not happy. They worship the non-existent, promote nihilism, and are killers of the 'ātman', the essence."

इत्युक्तत्वात्।

- because it is stated thus in the testimonial (Bhagavata Purana).

"महादुःखैकहेतुत्वात् प्राप्यत्वादसुरैस्तथा। असुर्या नाम ते लोका तान्यान्ति विमुखा हरौ॥"

"The demons acquire them (the demoniacal worlds), as they are the sole cause of the intense suffering. Those worlds are called 'Asurya', as those who turn away from lord Hari attain them."

इति वामने।

- stated thus in the Vāmana Purāna.

ये के चेति नियम उक्तः।

By stating 'ye ke' , i.e. 'those who', the governing principle is conveyed (i.e. those opposed to the lord mandatorily attain darkness).

"नियमेन तमो यान्ति सर्वेऽपि विमुखा हरौ।"

"By the governing principle, all those turned away from Hari attain darkness."

इति च ॥३॥

- there are such testimonials as well.