A yāga. Kings used to conduct this in order to get absolved of all sins. Fixing a victory card on the head of a horse it is allowed to roam about freely. If anybody stops the horse and ties it, the King should go and defeat him in battle and bring back the horse. If you do a hundred such yāgas you can become Indra.
Note: *) Aśvamedha or horse sacrifice was performed by autocrats to establish their supreme sovereignty. For this the horse let out to roam about should go into all countries. Those who opposed the sovereignty of the King could stop the horse and tie it. Then the King had to defeat him before conducting the yāga. The vedas enjoin that the sacrificial horse should be followed by a hundred young men ready to fight those who opposed the King. Indra had conducted a hundred such yāgas.]