One verse

Srimad Bhagavatam 6.17.38

6.17.38

जज्ञे त्वष्टुर्दक्षिणाग्नौ दानवीं योनिमाश्रित: । वृत्र इत्यभिविख्यातो ज्ञानविज्ञानसंयुत: ॥ ३८ ॥

jajne tvastur daksinagnau danavim yonim asritah vrtra ity abhivikhyato jnana-vijnana-samyutah

jajne
was born
tvastuh
of the brahmana known as Tvasta
daksina-agnau
in the fire sacrifice known as daksinagni
danavim
demoniac
yonim
species of life
asritah
taking shelter of
vrtrah
Vrtra
iti
thus
abhivikhyatah
celebrated
jnana-vijnana-samyutah
fully equipped with transcendental knowledge and practical application of that knowledge in life.

Being cursed by mother Durga (Bhavani, the wife of Lord Siva), that same Citraketu accepted birth in a demoniac species of life. Although still fully equipped with transcendental knowledge and practical application of that knowledge in life, he appeared as a demon at the fire sacrifice performed by Tvasta, and thus he became famous as Vrtrasura.