tathasuran avisad asurena
rupena tesam bala-viryam irayan
uddipayan deva-ganams ca visnur
daivena nagendram abodha-rupah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Thereafter, Lord Visnu entered the demons as the quality of passion, the demigods as the quality of goodness, and Vasuki as the quality of ignorance to encourage them and increase their various types of strength and energy.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Everyone in this material world is under the different modes of material nature. There were three different parties in the churning of Mandara Mountain — the demigods, who were in the mode of goodness, the demons, who were in the mode of passion, and the snake Vasuki, who was in the mode of ignorance. Since they were all becoming tired (Vasuki so much so that he was almost going to die), Lord Visnu, to encourage them to continue the work of churning the ocean, entered into them according to their respective modes of nature — goodness, passion and ignorance.