They (demons & demigods) were getting tired (Vasuki so much that he was almost going to die), Lord Visnu, to encourage them to continue churning the ocean, entered into them according to their respective modes of nature - goodness, passion and ignorance

Expressions researched:
"Since they were all becoming tired (Vāsuki so much so that he was almost going to die), Lord Viṣṇu, 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"

Srimad-Bhagavatam

SB Canto 8

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 Vāsuki, who was in the mode of ignorance. Since they were all becoming tired (Vāsuki so much so that he was almost going to die), Lord Viṣṇu, 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.

SB 8.7.11, Translation and Purport

Thereafter, Lord Viṣṇu entered the demons as the quality of passion, the demigods as the quality of goodness, and Vāsuki as the quality of ignorance to encourage them and increase their various types of strength and energy.

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 Vāsuki, who was in the mode of ignorance. Since they were all becoming tired (Vāsuki so much so that he was almost going to die), Lord Viṣṇu, 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.