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SB 1.13.49

so ’yam adya mahārāja
 bhagavān bhūta-bhāvanaḥ
kāla-rūpo ’vatīrṇo ’syām
 abhāvāya sura-dviṣām
Synonyms: 
saḥ — that Supreme Lord; ayam — the Lord Śrī Kṛṣṇa; adya — at present; mahārāja — O King; bhagavān — the Personality of Godhead; bhūta-bhāvanaḥ — the creator or the father of everything created; kāla-rūpaḥ — in the disguise of all-devouring time; avatīrṇaḥ — descended; asyām — upon the world; abhāvāya — for eliminating; sura-dviṣām — those who are against the will of the Lord.
Translation: 
That Supreme Personality of Godhead, Lord Śrī Kṛṣṇa, in the guise of all-devouring time [kāla-rūpa] has now descended to the earth to eliminate the envious from the world.
Purport: 

There are two classes of human beings, namely the envious and the obedient. Since the Supreme Lord is one and the father of all living beings, the envious living beings are also His sons, but they are known as the asuras. But the living beings who are obedient to the supreme father are called devatās, or demigods, because they are not contaminated by the material conception of life. Not only are the asuras envious of the Lord in even denying the existence of the Lord, but they are also envious of all other living beings. The predominance of asuras in the world is occasionally rectified by the Lord when He eliminates them from the world and establishes a rule of devatās like the Pāṇḍavas. His designation as kāla in disguise is significant. He is not at all dangerous, but He is the transcendental form of eternity, knowledge and bliss. For the devotees His factual form is disclosed, and for the nondevotees He appears like kāla-rūpa, which is causal form. This causal form of the Lord is not at all pleasing to the asuras, and therefore they think of the Lord as formless in order to feel secure that they will not be vanquished by the Lord.