The Pious mantra for Lord Krishna are
Om Kleem Krishnaya Namaha and
Om Namo Bhagawate Vasudevaya
The guards of Maha Vishnu - Jay and Vijay had to take birth as Rakshas on earth three times. First as Hirayanaksha who were destroyed by Bhagwan Narasimha, second as Ravana and Kumkharana who were killed by Lord Ram.
In Dwapar yuga, Jai and Vijay were born as Kamsa and Shishipal. They caused extreme pains to the people of Earth as well as to celestial beings including Lord Indra and others. To relieve the pains of all human and celestial being, Lord Vishnu took another incarnation and this time as Lord Krishna.
Shishupal was born with four hands and three eyes. Everyone was terrified to see the extraordinary child. As soon as Shishupal was born, an akashvani (celestial announcement) was heard that the child can only be destroyed by one person, who when cradles the baby, his extra arms and eye will vanish. The parents of Shishupal asked everyone to carry the newborn baby. Krishna and Balaram visited the parents of Shishupal and when Krishna picked up Shishupal, his extra arms and eye disappeared. Shishupal's parents requested Krishna to not kill their child. Krishna promised that he will forgive one hundred misdeeds of Shishupal. During the Mahabharata, Lord Krishna killed Shishupal with his Chakra after he committed one hundred misdeeds.
God Krishna used his divine play to destroy several rakshas like Kamsa, Bakasura, Putana, Saktasura, Kalia Nag and many more. He also guided the Pandavas to destroy evil such as Jarasandha and Duryodhana.
Lord Krishna also destroyed the egos of Indra, Varun and other Gods by lifting the mountain Govardhana to save the life of the people of Gokula. God Krishna married Rukmini, daughter of King Bhishm.
Once Krishna went to heaven, Radha, the daughter of Vrusbhanu and Kalawati, ran by Krishna to meet with her friends. Bhagwan Krishna felt the gust due to Radha's speed and raised hi eyes towards the movement. They both saw each other and felt extraordinary love at first sight and married immediately in an ancient method known as eye-union marriage. The Gopikas (milk girls of Gokula) were born from skin pores and therefore Krshna loved them just like Radha.
Krishna was born on the 8th day of the dark half or Krishna Paksha of the month of Bhadrapada in the Hindu Calendar, when the Rohini Nakshatra is ascendant. On Shri Krishna's birthday, one must remember Krishna, Balaram (Krishna's brother), Vasudev (Kriahna's Father), Devki (Krishna's Birth Mother), Yashoda (Krishna's Mother), Sudama (Krishna's friend) and Sandeepan (Krishna's guru). On his birthday, devotees offer flowers, pray next to a mountain and decorate the home/town/village and Temple.
Maha Vishnu came to Earth from Vaikuntha in complete Krishna form, showed his Maya and power from early childhood. Saints give the highest honor to Lord Krishna. Salvation cannot be attained from praying to any other Gods. Only by the divine blessings of Lord Krishna can one attain Salvation. Therefore, we must all pray to Lord Krishna if we desire Mukti.