The entire Hero's Path promotion, consisting of events at the prerelease, launch weekend, and Game Day for every set in Theros block, has been leading up to a single conclusion. Just as the storyline of the block culminates with a showdown against the ascended, power-mad Xenagos, so does the Path end with Journey into Nyx's Game Day and a challenge deck titled Defeat a God, which indeed pits players against Xenagos.
Now, the SocialMTG Tumblr blog has uncovered the package local game stores will be receiving to set up Defeat a God challenges. The box's most prominent image is the well-known picture of an enraged Xenagos facing off against the ragged figures of Elspeth and Ajani on top of the ruins of other gods' temples. The background of the package is a very Nyx-ish purple, accentuating the massive star in Xenagos' eye and the fact that the starfield has indeed consumed half his face.
The poster has a couple of readable words, which include "Xenagos" and "rollicking" - the expected descriptor of the Satyr God - but nothing about the actual contents of the Challenge Deck. How Xenagos will be represented as the opponent (he clearly is not multiple creatures, like the Hydra and the Minotaur Horde have been) remains to be seen.