The Invincible Demon
Tarakasura had performed tremendous tapasya and won a seemingly perfect boon: he could only be killed by the son of Shiva. At the time, Shiva was in deep meditation after Sati's death, and Tarakasura was confident no such son would ever exist.
He ravaged all the worlds. The devas were defeated and driven from their kingdoms. Even Indra could not stand against him.
This was why the gods needed Parvati to win Shiva — only their son could save creation.
The Birth of Kartikeya
When Shiva and Parvati finally united, the seed of Shiva was so potent that the celestial bodies themselves could not contain it. It passed through fire, through the Ganga, into the tall grasses of the forest. From those grasses, six infant forms emerged — the Krittikas, the Pleiades sisters, adopted each child.
The infants merged into one — a child with six heads and twelve arms. He was Kartikeya (also called Skanda, Murugan, Shanmukha).
The War
Kartikeya grew with terrifying speed. The gods gave him their weapons. Indra gave his thunderbolt. Yama gave his noose. The ocean gave a spear of cosmic power.
Vishnu came and said: "You shall be the commander of the gods."
The armies assembled. Tarakasura came with his demon host — and in the great battle that followed, Kartikeya slew him with his Vel — the divine spear — a single blow.
The gods returned to their kingdoms. Indra took back his throne.