This article contains some spoilers and references to Fantastic Beasts and Where To Find Them, so if you're trying to avoid any and all info about the film, read no further. We've been digging through the Harry Potter archives for any information available on Gellert Grindelwald, and there are some key details about the dark wizard to keep in mind. With that in mind, some of these details could prove to be pretty spoilery to the Fantastic Beasts films, assuming we're correct to believe Grindelwald will be a key character moving forward.
Grindelwald was expelled from Durmstrang at 16 years of age.
This piece of information is provided to us by Rita Skeeter, who penned a scathing book about Albus Dumbledore. Pretty much everything Rita Skeeter writes should be taken with a grain of salt. However, there is a bit more weight to Grindelwald's disgraceful exit from Durmstrang, in the form of a letter Dumbledore wrote to Grindelwald (also included in Rita Skeeter's book), which references a mistake Grindelwald made at Durmstrang, related to using force when it wasn't entirely necessary. That may have been Dumbledore's more forgiving assessment of the situation, considering Rita Skeeter's account mentions something to do with twisted experiments. I'd say the odds are high there's truth somewhere between Dumbledore's account, and Skeeter's. Beyond whatever went on that got him expelled, Grindelwald also carved the Deathly Hallows symbol in a wall at Durmstrang, and it was still there as of the time Viktor Krum attended school there.
He was only friends with Dumbledore for a couple of months.
Grindelwald showed up in Godric's Hollow not long after Dumbledore's mother Kendra died. At this point, Dumbledore was fresh out of Hogwarts and supposed to be traveling around the world with his school friend, Elphias Doge, but after his mother's death, he stayed behind to look after his sister, Ariana. When Grindelwald arrived to stay with his great-aunt Bathilda Bagshot, he and Dumbledore became fast friends. It sounds like they bonded over their mutual frustration over the international Statute of Secrecy, sharing a similar goal to see wizards rule over muggles, "for the greater good." Dumbledore describes the friendship as two months of insanity and cruel dreams. He also blames himself for the neglect he showed to his brother Aberforth and sister Ariana that summer.
Grindelwald used the Cruciatus curse on Dumbledore's brother.
Fed up with Dumbledore's neglect of their sister, and not one to stay out of it when he has something to say, Aberforth Dumbledore confronted his brother about his concerns over Dumbledore's plans to travel and search for Hallows with Grindelwald. The confrontation led to Grindelwald getting in Aberforth's face, and the two began to quarrel, leading to Grindelwald using the cruciatus curse on Dumbledore's brother. Albus got involved from there, and a fight broke out, which ended with Dumbledore's 14-year-old sister Ariana dead. Grindelwald fled after that, and as far as we know, that was the end of his and Dumbledore's friendship.
Grindelwald got the Elder Wand sometime after he met Dumbledore
Dumbledore doesn't get specific on the timeline, so we don't know if Grindelwald has the Elder Wand during the events of Fantastic Beasts and Where To Find Them (assuming the wand Graves was using belongs to the actual Percival Graves). What Dumbledore does tell Harry is that there were rumors about Grindelwald acquiring a wand of great power in the years after he and Dumbledore were friends. In Deathly Hallows Part 1, Grindelwald looks pretty young in Gregorovitch's memory of Grindeldwald stealing the wand, however the movies can't always be relied upon for details like that. What we do know is that Grindelwald will get the wand from Gregorovitch at some point, and Dumbledore will win the wand from Grindelwald in 1945.
Grindelwald probably knows who killed Ariana Dumbledore
Ariana Dumbledore was killed during the fight between Dumbledore, Aberforth and Grindelwald. Neither Dumbledore nor Aberforth were sure which of the three of them fired the spell that killed her. Dumbledore admitted to Harry in Deathly Hallows that he was terrified of Grindelwald after his sister's death, but not because he feared Grindelwald's power. Dumbledore was actually afraid Grindelwald would tell him that he, Dumbledore, was the one who struck the magical blow that killed his sister. That is Dumbledore's explanation for why he kept his distance from Grindelwald for years. That seems particularly relevant when taking Fantastic Beasts into account, as it may explain Dumbledore's assumed lack of involvement in Grindelwald's rise to power.
He kept his reign of terror away from the U.K.
Among the information provided by Rita Skeeter (insert small grain of salt here), is the fact that Grindelwald apparently "never extended his reign of terror to Britain," which may explain why there isn't a lot of information on him in the U.K.. We're going to give Rita Skeeter the benefit of the doubt on her info about Grindelwald's absence in the U.K., as there really isn't any other information that contradicts it. This is not to say he never comes to the U.K., but if he does, it sounds like it was undocumented (or potentially obliviated). Regardless, it might be safe to assume Grindelwald is giving Dumbledore a wide berth. In which case, smart man.
Grindelwald may want to build an army of Inferi
Dumbledore confides in Harry that he was pretty sure the only reason Grindelwald sought the resurrection stone was to create an army of Inferi (magically reanimated corpses). That's information that could prove to be particularly relevant, assuming Grindelwald ever manages to get his hands on that stone. To our knowledge, the resurrection stone was set in a ring and kept in the possession of the Gaunt family until Tom Riddle came calling and took it. So it doesn't seem likely that Grindelwald will ever track it down. (Of course, it's always possible he finds a way to "borrow" it for a little while.)
It was a "dreadful" moment when Dumbledore finally decided to confront Grindelwald
In a passage about Minerva McGonagall, J.K. Rowling reveals that, after a bonding heart-to-heart between McGonagall and Dumbledore, Dumbledore told the transfiguration professor about the dreadful moment when he decided to confront and defeat Gellert Grindelwald. The fact that Dumbledore only shared this information with a handful of people suggests that the choice to face his old friend was a major moment in his life, and one he preferred not to talk about under normal circumstances. Given the circumstances, and what he feared he would learn, it's completely understandable.
Dumbledore's triumph over Grindelwald is a turning point in magical history.
The source is a bit suspect on this piece of information, as it's pretty evident that Elphias Doge thought very highly of Albus Dumbledore. So he may have been exaggerating just a tad when he spoke of his old school friend's accomplishments in Dumbledore's obituary. Still, it's fair to say that the duel between Dumbledore and Grindelwald is a big deal, so it's not entirely hard to believe that it would have been a turning point in magical history. How, exactly, is unclear, but the fight was one of Dumbledore's greatest known achievements, and it sounds like it had pretty big consequences on the wizarding world.
Grindelwald died after lying to and laughing in Voldemort's face
In Deathly Hallows, Harry mind-flashes to Voldemort demanding information from a skeletal Grindelwald in prison. Grindelwald lies to Voldemort, claiming he never had the wand in the first place, and then laughs in his face, clearly unconcerned with the Dark Lord's wrath. Here were his final words, before Voldemort killed him:
For reference, the Deathly Hallows movie shows this scene very differently. Not only does Grindelwald have way more teeth than he's described in the book, but the movie also has him willingly handing over the information to Voldemort. We're using the book's version as canon, and in Rowling's take on the events, Grindelwald doesn't betray Dumbledore or give up any information. Dumbledore later speculates that Grindelwald lied to keep Voldemort from getting the Hallow, though Harry thinks that maybe Grindelwald was trying to prevent Voldemort from raiding Dumbledore's tomb. Either way, it's suggested by Dumbledore that Grindelwald showed remorse for his misdeeds in his later years, which certainly sets him apart from Lord Voldemort as a villain.