How did lord Krishna describe about Kali Yug?
According to lord Krishna, What is Kali yug? and what will happen during Kali yug?
The only place I know of where Krishna discusses the nature of the Kali Yuga is in this chapter of the Srimad Bhagavatam, as part of the Uddhava Gita:
O saintly Uddhava, in the near future I will abandon this earth. Then, being overwhelmed by the Age of Kali, the earth will be bereft of all piety. My dear Uddhava, you should not remain here on the earth once I have abandoned this world. My dear devotee, you are sinless, but in Kali-yuga the people will be addicted to all types of sinful activities; therefore do not stay here.
Also, in this chapter of the Udyoga Parva of the Mahabharata, Krishna uses the Kali Yuga as a metaphor for how bad it will be to face the Pandavas in battle:
When thou wilt behold in battle Arjuna, on his car drawn by white steeds and driven by Krishna, applying Aindra, Agneya and Maruta weapons, and when thou wilt hear the twang of Gandiva piercing the welkin like the very thunder, then all signs of the Krita, the Treta, and the Dwapara ages will disappear (but, instead, Kali embodied will be present). When thou wilt behold in battle Kunti's son, invincible Yudhishthira, devoted to Yapa and Homa and resembling the very sun in brilliance, protecting his own mighty army and burning the army of his foes, then all signs of the Krita, the Treta, and the Dwapara ages will disappear. When thou wilt behold in battle the mighty Bhimasena dancing, after having quaffed the blood of Dussasana, like a fierce elephant with rent temples after having killed a mighty antagonist, then all signs of the Krita, the Treta, and the Dwapara ages will disappear. When thou wilt behold in battle Arjuna checking Drona and Santanu's son and Kripa and king Suyodhana, and Jayadratha of Sindhu's race, all rushing fiercely to the encounter, then all signs of the Krita, the Treta and the Dwapara ages will disappear. When thou wilt behold in battle the two mighty sons of Madri,--those heroic car-warriors, capable of breaking into pieces all hostile cars,--agitating, from the very moment when weapons will begin to clash, the army of Dhritarashtra's sons like a couple of infuriated elephants, then all signs of the Krita, the Treta and the Dwapara ages will disappear.