What did Nanahuatzin and Tecuciztecatl agree to do?

Nanahuatzin and Tecuciztecatl agreed to sacrifice themselves by jumping into a ceremonial fire as an offering to the gods in order to bring light back to the world.

Nanahuatzin and Tecuciztecatl agreed to sacrifice themselves in order to become the new sun and moon, respectively, during the creation myth of the Aztec civilization.