Camazotz

He’s a Mayan bat god who dwells in the underworld.

In Fate/Grand Order Cosmos in the Lostbelt: Six million years prior to the modern day of the South American Lostbelt, Camazotz was king of the Ka'an Kingdom that came into existence during the period in which Mictlan had no source of light. After ORT began to awaken, the humans living there sacrificed themselves to empower their king, who became unkillable. In the present day Camazotz is feared by the deinos as a monstrous being who can raise the dead.