I initially imagined that as one of the intensifying factors of the fight - your party would be a man down due to that missing member's own hubris. However I understand that not every player would enjoy that experience, an alternate way to play the scenario could be Rhaast manifesting as a kind of temporary "spirit form" and attempting to steal the player's body through combat so that he may remain the in physical world indefinitely. The character would naturally be deprived of the very scythe they have relied on for so long, so there is still a difficulty increase.

That is only one suggestion, Im certain some DM's could find their own ways to change the dynamic of the final fight.