From Blaseball Wiki
Phoenix is a Player Modification first seen in Pre-History I, which was first revealed in the Library when it was revealed in Season β21.
Players with the Phoenix modification will gain a 10 to 20% boost to their stats when they exit the Hall of Flame
As of the end of the Expansion Era, only one player has been seen to have the Phoenix modification. This player is Pre-History New York Millennials player Clare Ballard. They played for the Millennials during Pre-History, although at this time it is unknown if Ballard ever got to use their Phoenix modification.