Birth of Kartikeya

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 transfered 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 calledSkanda) 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. Shivagranted her this boon.

As per a prior prophesy, the child Kartikeya was anointed as the command-in-chief of the Devas.

This is one story, there are other myths also.  

Kartikeya is also known as Subramaniam, Skanda, Guha, Shadannana or Shanmukha (because he has six faces). He represents a person of perfection. He is the war God, the Hindu equivalent of Mars and Aries. He was created by all the Gods to lead the heavenly hosts and destroy the demons. He is the most masculine and fierce of all the Gods. While Ganesh removes all obstacles, Skanda bestows all spiritual powers, particularly the power of knowledge.

The Atharva Veda describes Kumaran as 'Agnibhuh' or son of Agni, the fire god. The Satapatha Brahmana refers to him as the son of Rudra and the ninth form of Agni. The Taittiriya Aranyaka contains the Gayatri mantra for Shanmukha. The Chandogya Upanishad refers to Skanda as the "way that leads to wisdom". The Baudhayana Dharmasutra mentions Skanda as 'Mahasena' and 'Subrahmanya.' The Aranya Parva canto of the Mahabharata relates the legend of Kartikeya Skanda in considerable detail. The Skanda Purana is devoted to the narrative of Kartikeya.^[8] The Upanisahds also constatntly make a reference to a Supreme Being called Guha, the indweller.