Pinak is the name of the bow of Shiva and this is the reason why he is known by the name Pinaki. He used the bow to annihilate three demons that had wrecked havoc in the world. The three demons – Vidyumali, Tarakasha and Viryavana – had propitiated Brahma and as boon asked for immortality which was denied and later they asked for three fortified cities which could fly around and could only be destroyed with one shot of an arrow.
Shiva agreed to put an end to the rule of the flying cities. Vishwakarma built the chariot for the mission. The Charioteer was Brahma. Earth was the body of the chariot. Sun and Moon where the wheels of the chariots. The four Vedas became the four steeds.
It is said that the Mount Meru became the shaft of Pinak and Sheshnaag became the bowstring. Lord Vishnu became the arrow.
Shiva destroyed the demon cities with the Pinak bow, when it aligned in a single line for a split of a second.