Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 24
स त्वं प्रभोऽद्य वसुदेवगृहेऽवतीर्ण:स्वांशेन भारमपनेतुमिहासि भूमे: ।अक्षौहिणीशतवधेन सुरेतरांश-राज्ञाममुष्य च कुलस्य यशो वितन्वन् ॥ २४ ॥
sa tvam prabho ’dya vasudeva-gṛhe ’vatīrṇaḥsvāṁśena bhāram apanetum ihāsi bhūmeḥakṣauhiṇī-śata-vadhena suretarāṁśa-rājñām amuṣya ca kulasya yaśo vitanvan
You are that very same Supreme Person, my Lord, and You have now appeared in the home of Vasudeva with Your plenary portion. You have done this to relieve the earth’s burden by killing hundreds of armies led by kings who are expansions of the demigods’ enemies, and also to spread the fame of our dynasty.
The term suretarāṁśa-rājñām indicates that the demoniac kings slain by Kṛṣṇa were in fact expansions or incarnations of the enemies of the demigods. This fact is elaborately explained in the Mahābhārata, which reveals the specific identities of the demoniac kings.