Kingdom Hearts Birth by Sleep
In the beginning of Kingdom Hearts Birth by Sleep, Master Eraqus prepares to hold the Mark of Mastery exam for Keyblade wielders and apprentices, when he is introduced to a young Ventus after Eraqus's long-time colleague Master Xehanort arrives at the Land of Departure to witness Terra and Aqua as they endure their Exams. The exam is carried out soon afterwards, Aqua passing while Terra fails, Eraqus believing that Terra is unqualified on the grounds that he is unable to control his darkness. Little does Eraqus know that Master Xehanort had used his dark powers to tamper with the course of the Exam itself.
Soon after, Master Xehanort goes missing. Eraqus sends Terra and Aqua to find him shortly after being contacted by Master Yen Sid about the spawning of the mysterious Unversed. Sensing a possible connection between the two events, Eraqus later instructs Aqua to look after Terra, as he fears the man may be tempted by the darkness. However, after Terra departs, Aqua and Eraqus witness Ventus leaving as well, and he instructs Aqua to bring Ventus back.
As he reaches the end of his journey, Ventus discovers the truth about his past, and in order to get his questions answered, travels back to the Land of Departure so he may talk with Master Eraqus. Meanwhile, Terra receives this news from Master Xehanort and also learns that Eraqus plans to eliminate his friend, the man tricked into thinking that the only way to save Ventus from his grim fate being to eliminate his master. Terra arrives just before Eraqus is about to vanquish Ventus, Eraqus sensing that Terra has indeed descended into darkness. Seeing Terra's stained heart, he recalls a long-ago discussion between himself and Master Xehanort over the Keyblade War during which Eraqus disagreed with Master Xehanort's hypotheses and wanted to fight Master Xehanort for it. Master Xehanort responded by attacking Eraqus and leaving him with the scar he presently bears.
Greatly saddened by the fact that he must use force to eliminate two of his apprentices and even angered when Terra refuses his orders to stand down, Eraqus charges forward only for Ventus to be thrown into a portal to another world by Terra. Terra no longer caring that Eraqus is his master and someone he holds dear, Eraqus battles him. Terra wins the battle and leaves Eraqus wounded, but as both begin to show remorse for their actions (Terra for fighting his father figure and Eraqus for turning his Keyblade on Terra and Ventus in the first place), Eraqus is ambushed and finished off by Master Xehanort himself, and fades out of existence. He is later mentioned during the final battle between Terra and Master Xehanort, with Terra alluding to Eraqus as his father-figure and primary motive for defeating Master Xehanort.
Some time after Terra's possession at Master Xehanort's hand, their hearts begin to converse with each other. Master Xehanort then realizes that Eraqus's heart now resides within Terra, with the two beginning to debate over who would control it.