"To shine your light, both within and without, to be never cruel, never cowardly, to give hope where there was none, to fight for peace and justice, to never betray the hopes placed upon you, to never give in to the darkness to never give up. That is the Promise of Ultra, since before there were Ultras in the universe..."
The Vow is said to be the last words of the Heroes of Light before their spirits moved on and the gave up the last of their power to create a source of justice in the Multiverse. It is said that soul of every Ultra agrees to the oath before they are born. In the Land of Giants it is treated as the motto of the Space Garrison and every Space Ranger is expected to know it by heart.
In the Orion Universe, the title Ultraman is something taken up by all Space Rangers upon graduation. The title means that they have sworn to uphold the tenets of the Vow of Ultra and not just Galactic Law.
Upholding the vow is the reason for the existance of Ultras in the Fan Multiverse, when Ultras actively work to uphold it they may find that they gain unexpected strength in the heat of battle.