Conversation so far:
Who was Julian in relation to Constantius II?
Julian was Constantius's last surviving cousin.
Who promoted Julian to Caesar under Constantius II's time as Roman Emperor?
In 355, Constantius promoted his last surviving cousin Julian to the rank of Caesar.

Question: Who was the first usurper to try to take power from Constantius II?
Self-contained rewrite: Who was the first usurper to try to take power from Constantius II?