Takṣaka flattered Kaśyapa by presenting him with valuable offerings and thereby stopped the sage, who was expert in counteracting poison, from protecting Mahārāja Parīkṣit. Then the snake-bird, who could assume any form he wished, disguised himself as a brāhmaṇa, approached the King and bit him.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तं तर्पयित्वा द्रविणैर्निवर्त्य विषहारिणम् । द्विजरूपप्रतिच्छन्नः कामरूपोऽदशन्नृपम्
taṃ tarpayitvā draviṇairnivartya viṣahāriṇam | dvijarūpapraticchannaḥ kāmarūpo'daśannṛpam
SB 12.6.12