Skanda: The Warrior Son with Six Faces
Born of Shiva's fire, nursed by six Krittikas
A demon named Tarakasura cannot be killed by anyone except a son of Shiva, and Shiva is in deep tapas with no son. The Shiva Purana's answer is the strangest birth in Hindu literature. Shiva's seed, too hot for any single womb, is carried by Agni, cooled by Ganga, and nursed by six star-mothers. The boy who emerges has six faces and the strength to end what nothing else can touch.
The Demon Who Could Not Be Killed
It is years after the burning of the Kamadeva ashram and the wedding of Shiva and Parvati on Mount Kailasa. The cosmos should be peaceful. It is not.
A demon named Tarakasura has done the kind of tapas that bends Brahma's arm. He has stood in the sun, on the heat of five fires, for centuries. When Brahma finally appears and offers him a boon, Tarakasura asks for the cleverest immortality in the Puranas. He does not ask to be unkillable. He asks to be killable only by a son of Shiva.
Brahma grants the boon. The demon laughs. Shiva is in the deepest tapas of his life, mourning Sati, ash on his body, eyes closed on Kailasa. The marriage with Parvati has not yet produced a child. The marriage may not produce one for another eon. By the time Shiva has a son, Tarakasura will have eaten the world.
He sets to work. The demon takes Amaravati, the city of Indra. He takes the wealth of Kubera. He takes the cattle of the rishis. The gods flee. Indra hides. The Vedic order, the yajnas, the seasonal rains, the river flows, all begin to slow. The Shiva Purana describes the situation with a single hard sentence. Without a son of Shiva, the world is finished.
The Plan
The gods go to Brahma. Brahma sends them to Vishnu. Vishnu names the only path. Shiva must wake from tapas. Shiva must marry Parvati. Shiva must have a son.
The earlier lesson on Kamadeva gave the first half of this answer. Kama was sent to wake Shiva, was burned to ash for his trouble, and the desire that could move Shiva was finally lit by Parvati's own tapas. The wedding has happened. Yet still, the gods worry. Shiva and Parvati's union is not an ordinary household conception. Their seed is the seed of the universe. No womb can hold it. No fire can contain it.