Kʼinich Janaab Pakal I was an ajaw (king) of the Maya kingdom of Palenque.
Behind the Scenes
Pakal being an "ancient astronaut" is a direct reference to the "Ancient Astronaut" conspiracy theory popularized by the novel Chariots from the Gods and brought to the mainstream due to the History Channel series, Ancient Aliens and its spin-off, In Search of Ancient Aliens. Needless to say, Pakal being an astronaut is not accepted by the vast majority of Mayanist, as his relief instead contains several consistently appearing iconography symbolizing death and after life in Maya society.
He appeared in the story, Gift of the Sun Lord