The person who entered the mobile fortress while it was battling the eldritch beholder was Speedrun, the player who possessed the Master Thief special class. When he was killed by Hatori, he was wearing an amulet of rebirth. Speedrun always wore this amulet all the time. He wasn't a combatant. He didn't care about the ability boost provided by a piece of pendant equipment. Keeping his level and belonging was more important to him.
Speedrun dropped one level to level 75 after he died, but Master provided him with an exp-grinding team to power-level him. Master even helped by being in that team and letting Speedrun enter a special legacy dungeon in their guild headquarters which provided tons of exp points. Normally, only he and a couple of his high officers were allowed to use this legacy dungeon.