Kingdom Hearts Union Cross quest 970 reveals that Brain and Eraqus share an interesting connection through their Keyblades. During the quest, Brain pulls his weapon for the first time to start the final battle between the Dandelions and Darkness, and in doing so, he reveals that he and Eraqus both use the Master Keeper Keyblade. While it's currently unclear what exactly Brain's relationship with Eraqus is, Kingdom Hearts Dark Road and beyond may imply that he's Eraqus' fabled grandfather.
Eraqus' granddad is a character first alluded to in Kingdom Hearts Dark Road. While the game doesn't reveal his identity, some hints sprinkled throughout modern Kingdom Hearts titles suggest that he could be Brain, especially now that Kingdom Hearts Union Cross has revealed that Brain chronologically wields Master Keeper before Eraqus. Three hints, in particular, stand out that support this theory, with the first being:
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Eraqus knows Brain's "Darkness can hide anywhere, even inside someone" quote verbatim. He says it in Dark Road episode 2, "The Presence of Darkness," after he and his friends, including Xehanort, escape execution by the Queen of Hearts in Wonderland. Both Eraqus and Xehanort say it simultaneously to present their theory on why the Queen of Hearts acts as volatile as she does, and Eraqus seems surprised that Xehanort knows it too. He reacts almost as if Xehanort shouldn't know it since Eraqus had to learn it from Brain himself, which would be possible if Brain was his grandad.
Of course, Xehanort and Eraqus both could've just learned this quote from their teacher, Master Odin, as he's knowledgable enough about darkness to have been able to say something akin to the quote to Eraqus' whole sibling-apprentice group. In which case, Eraqus was probably just surprised that Xehanort was not only thinking the same thing he was but that he said it at the same time, in the same manner. However, Dark Road makes sure to imply that this isn't the case and that Eraqus really is just surprised that Xehanort knows the quote at all.
During the Dark Road's present-day storyline, Xehanort is experiencing the memories of someone from Union Cross' time, or at the very least, is perceiving events from that time for one reason or another. As a result, right before he says the quote, he physically remembers Brain saying it while discussing Ventus and Strelitzia with Ephemer, Skuld, and Ven himself. If Xehanort learned this quote from Odin, there'd be no reason for Dark Road to have him remember it from Brain.
Another hint labeling Brain as Eraqus' grandfather is that they both have notable battles against the darkness. Dark Road episode 3, "The Purpose of the Journey," brings up Eraqus' grandfather's fight against darkness during its epilogue featuring Eraqus and Bauldr. The battle's exact details didn't come up, but it's clear that Eraqus' grandfather went up against a powerful dark enemy and that the confrontation is a famous one.
Overall, the situation is very similar to what Brain is about to experience in Union Cross. As mentioned, Brain is about to go up against darkness itself, and the battle is considered to be the last major hurdle the Dandelions will have to cross before they can restore the world following its destruction in the Foreteller's Keyblade War. Simply put, it's the kind of spectacle that would become famous after the fact, just like whatever battle Eraqus' grandfather went through.
The Dark Road episode also makes sure to disguise what happens to Eraqus' grandad after the fight. Bauldr alludes to him dying, but Eraqus reacts as though something else happened that he doesn't want to discuss. Dark Road is obviously doing this because the reveal of what happened will be an important element later on in its story, but it could also be doing it to avoid spoiling Brain's fight, as it will be a part of Union Cross' ending.
The last hint actually comes from Kingdom Hearts 3. Like most other titles in the series, players can complete various activities in KH3 to collect things called Secret Reports, which provide more insight into the game's plot, characters, and future plotlines. The "Secret Report 2: Mark of Mastery Journal" is written by Xehanort, and in it, he mentions that Eraqus is a blueblood who descended from the very first masters of Union Cross' time. In other words, from the sounds of it, Eraqus is a descendant of one of the Union leaders.
Union Cross has twelve notable Keyblade Masters: the Master of Masters, the Master's six apprentices, and the original five chosen replacement Union leaders. The Master of Masters and his apprentices can be ruled out immediately since they disappeared during the Keyblade War and left rebuilding the new world to the replacement Union leaders.
Of the five replacements, Strelitzia died right out of the gate, and Lauriam is confirmed to have traveled from the past into the future, leaving neither of them time to have kids. Since Skuld is a girl, she would make for a famous grandmother — like Kairi's — rather than a famous grandfather. Of course, this leaves only Ephemer and Brain, and out of these two, Brain is the only one wielding the same Keyblade as Eraqus.
Of course, the identity of Eraqus' grandfather isn't a confirmed thing yet, so the idea that it's Brain is just that: an idea. While Brain wielding Eraqus' Keyblade makes him a prime candidate for the role, nothing says that Master Keeper is a family heirloom. Eraqus actually claims in Birth by Sleep that, after he became a master, his teacher gave him the Keyblade along with the responsibility of protecting the Land of Departure. He even alludes that generations of Keyblade wielders passed the weapon down via master and student, not familial connection.
If Brain turns out to be Eraqus' grandfather, Union Cross will likely reveal it in its upcoming Final Chapter. If Union Cross's Final Chapter doesn't do it, then, at the very least, it'll show how the Keyblade leaves Brain's hands and enters the passing-down cycle that eventually leads it to Eraqus. Eraqus' granddad's reveal will then happen sometime later in Dark Road itself, whether it be Brain or someone else.
Kingdom Hearts Union Cross is on IOS and Android.
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