We first see her in book 7 near the end. She transports Peggy, and Luna Mallop to a fight. in the end Luna goes blind from some kind of toxic goo and Marxeen dies.
A month later she comes back. The Carrows have the ablity to die and come back to life, (only once. She is made the leader of Big Ben clock tower in "Persoina." Their she plots to overtrow the Carrows to make her the leader, by using an angle named Faith, and taking the memories of her other. She made a grave mistake when she slapped Faith. Making Peggy and a robot named Noodle, who was under her control join up and attack her. The robot was "killed" and shortly after Peggy killed Marxeen for the last time.
She was seen after that for a moment in book 8, the nothingness, is were the Carrows go after they die for the last time. the leader, Malcom found out her plan and killed her. Making her dissaper for ever
Marxeen has light/pale pinkish yellow hair and weilds a huge scythe with and eight foot blade.