There are many candidates for fallen Jedi the title of the fourth Ahsoka episode refers to. There is Baylon Skoll, the antagonist. It could mean Ahsoka herself, since she explicitly rejects the title. It might even be Anakin Skywalker, who Ahsoka sees at the episodes climax when she enters into the mysterious World Between Worlds. One person it isn't, however is the mysterious Sith Inquisitor, a dark side group comprised of fallen Jedi. The fate of Marrok as "just some guy" is an inspired choice that will almost certainly disappoint Star Wars fans who had theories.
Source：https://www.cbr.com/marrok-fate-ahsoka-disappoint-fans/
