Jeanne of vengeance, a "draconic witch who rose from her grave to avenge herself upon France" created by Gilles de Rais using the Holy Grail. While referred to as "Alter," she is not another "aspect" of Jeanne, but rather she is a creation of the Holy Grail born from Gilles de Rais' wishes shortly after the execution of Jeanne in 1431, an existence encapsulating much of the corrupted Gilles' biases and feelings.

(Source: TYPE-MOON Wiki)