Eson is the Celestial revealed inside the Ship in X-Men '97 season 2.
The discovery occurs during the X-Men's journey through ancient Egypt, where they search for a temple containing cosmic power. That temple turns out to be Ship, the advanced vessel later associated with Apocalypse. En Sabah Nur sees visions of the power awaiting him and accepts it from Eson, beginning his transformation into Apocalypse.
Eson is one of Marvel's Celestials, immense cosmic beings connected with the creation and development of life across the universe. The character is often confused with Arishem, another Celestial who is more prominent in the Marvel Cinematic Universe, or Tiamut, the dormant Celestial at the center of Eternals. In this storyline, however, the named Celestial is specifically Eson.