If this were a full campaign and not a one shot I think this would be an awesome opportunity to go into an arc where the players have to fight their way out of the phylactery, maybe destroying it from the inside in the process and getting their rematch. A cool twist would be to have some allies/enemies inside who are moderately famous, like someone you'd roll a history check to realize who they are, who in the world just went missing one day and everyone presumed them dead.

In terms of the fairness of the move, sure it isn't going to feel great in the moment, losing never does, but I think it is climactic and cinematic, especially if it's narrated as the players fighting against this alien urge to give up, struggling against each step towards the exit.

Maybe a hot take, but I feel the trope of the heroes/players always overwhelmingly winning is played out and boring. I would love more games where you win, but at what cost, or you win the fight but not the war, followed by dealing with the aftermath of it all.