After their marriage, Lord Shiva and Parvati lived happily at Mt. Gandhamadana. Once, while they were enjoying an intimate moment, a small quantity of Shiva‘s vital fluid fell on the ground. Tremendous amount of heat began emanating from it and it threatened to engulf the entire world in flames. Acting on the advice of Brahma and Vishnu, Agni went there in the guise of a mendicant and devoured the vital fluid.
Parvati felt insulted. She cursed Agni saying, “From this day thou shall be omnivorous and eat unclean things. The vital fluid that you have devoured shall cause an unbearable inflammation on your body.” (Note: See this story for an alternate curse for why Agni became omnivorous.)
Instantly, Agni got unbearable inflammation on his body. He beseeched Lord Shiva for a way to end his agony. Lord Shiva said, “You will be relieved of this affliction when you transfer my vital fluid to the womb of a woman.”
Agni retired to a desolate place, waiting for a suitable woman to arrive. He beheld the six Kritikas, all shivering with cold who arrived there. He then transferred Shiva‘s vital fluid to their wombs. They became pregnant. When their husbands came to know of this, they cursed the six women, to be turned into stars in a constellation in the sky. Before the transformation, the Kritikas aborted their fetuses on the Himalaya mountains. The holy river Ganga carried the fetuses to a secluded place, called the Sara Vana, which was covered with reeds. After some time, Lord Kartikeya (also called Skanda) manifested from those reeds. Since he was born to six mothers, he had six heads. Since he was born in the Sara Vana, he is also known as Saravana.
[Note: The Six Kritikas are the stars of the constellation pleiades. In other texts, Agni carries the six sparks that fell from the third eye of Shiva, not his vital fluid. The sparks are deposited in a pond in the Sara Vana, where a six babies are born on a lotus each. The Krittikas who came there, nursed the babies. When Parvati and Shiva came there, Parvati took all the babies in her arms and made them into one, albeit with six heads.]
The Devas rejoiced at the birth of this child. Lord Shiva and Parvati went to the place where the child was born. Parvati prayed to Shiva that the child may no her and none other as his mother. Shiva granted her this boon.
As per a prior prophesy, the child Kartikeya was anointed as the command-in-chief of the Devas. It had been foretold that only he could kill a demon named Taraka who had been plaguing the Devas. However, that incident is part of another story.
[Source: www.apamnapat.com]