It’s Eris Boreas Greyrat at the end of the Mushoku Tensei season 3 trailer, shown in a throne-like pose as the big reveal for her return. The strongest support for that is the Season 3 trailer coverage and breakdowns that specifically identify the ending as Eris’s comeback and the Sword King teaser.

Why people think that

  • Trailer discussions consistently point to the final shot as Eris , not a new character.
  • The official-season coverage notes the trailer’s focus shifts toward her return after the family-life scenes.
  • Fan analysis pages also describe the ending teaser as Eris’s reveal in her new, stronger form.

What the scene suggests

The throne-style framing seems meant to emphasize her upgraded status and dramatic reintroduction, not necessarily that she is literally ruling anything. In other words, it’s a character reveal shot more than a plot confirmation.

TL;DR

The person in the throne at the end is very likely Eris.