From an absolute point of view, Chenrezig is without origin; he exists primordially. However, from the relative point of view, there is a beginning of his manifestation in the realm of phenomena ...
Amitabha, the Buddha of Infinite Light who reigns in the Land of Bliss (Dewachen), one day conceived that in order to help beings a deity in the form of a young man should be manifested. His right eye then emitted a beam of white light that took the form of Chenrezig. He saw the necessity of having a deity in the form of a young woman and a beam of green light spread from his left eye giving birth to Tara ....
When Chenrezig ... looked at beings with compassion, he saw that they were covered with many karmic veils formed by the influence of desire, aversion, blindness, jealousy, and pride. Thus their sufferings were immeasurable. He saw all of that and a tear dropped from each of his eyes. Tara appeared from the tear that dropped from his right eye and the goddess Lhamo Trulnyerchen appeared from the tear that fell from his left eye. The two deities turned toward him and said, "Do not be scared. We will help you with your mission to benefit beings." Then suddenly they melted again into his eyes.
When he was in the presence of Amitabha, Chenrezig thought, "As long as there is even one being who has not attained awakening, I will strive for the benefit of all. And if I break this promise, may my head and body split into a thousand pieces!"
Amitabha understood his thought and told him, "This promise is excellent. Myself and all the buddhas of the three times, having taken such commitments, attained awakening for the benefit of all. I will help you to accomplish that which you have promised." Chenrezig's body then emitted six beams of light that produced emanations whose destiny was to act for the benefit of all in each of the six realms of being: humans, gods, demigods, animals, hungry ghosts, and hell beings.
He thus worked for many kalpas [eons]. Then one day, he looked with the eye of knowledge from the top of mount Meru [the center of the Universe] to see if he had liberated many beings and if the number of beings in samsara [cyclic existence] had diminished. Alas, he saw that they were still innumerable.
He was very sad. Being discouraged, he thought, "I do not have the capability to help beings; it is better that I rest in nirvana [liberation from cyclic existence]."
This thought contradicted his promise, and he burst into a thousand pieces and felt intense suffering.
Amitabha, by the power of his grace, reconstructed the body of Chenrezig. He gave him eleven faces and a thousand arms similar to the thousand spokes of the universal monarch's wheel and a thousand eyes, symbolic of the thousand buddhas of the present kalpa. Chenrezig could henceforth help the beings in this form as well as with his other forms of two or four arms. Amitabha asked Chenrezig to take his promise with still more vigor than before and then transmitted to him the six syllable mantra: OM MANI PADME HUNG.
This is the story of Chenrezig's manifestation in the relative domain.