Ganesha and Gajamukha

Gajamukha was a wicked asura. He worshipped Shiva for years until Shiva granted him a boon. "Make me invincible to all weapons!" demanded Gajamukha.
Gajamukha then conquered the people and the gods, commanding them to worship him, and only him.
The gods fought back, attacking Gajamukha with arrows, spears, swords, clubs... nothing worked.
Ganesha then broke off a tusk and stabbed Gajamukha, wounding him badly. Gajamukha turned himself a mouse, but Ganesha sat on him, crushing the asura beneath his weight.
When Gajamukha begged for mercy, Ganesha forgave him, and so Gajamukha  became the mouse that Ganesha rides even now.

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Inspired by: Stories of Lord Ganesha
Notes: There is also a very different story about a musician-god named Krauncha who was cursed to become a mouse and eventually became Ganesha's vehicle.

(illustration by John Batten)