Years before Voldemort began his reign over the magical folk of Harry Potter, the British magical community was terrorized by the Dark Wizard Grindelwald, who made his first appearance in the Fantastic Beasts trilogy. Voldemort and Grindelwald had similar goals about Muggle subjugation and pure-blood supremacy, and it wouldn't have been surprising if they teamed up to take over the world together. They were both still alive at the beginning of the original Harry Potter franchise, so this has fans wondering if the pair of evil wizards ever met each other.
Albus Dumbledore defeated Grindelwald in 1945, and the First Wizarding War against Voldemort took place in the 1970s, so they definitely could have met at some point. Grindelwald had been in prison for about 53 years by the time Voldemort was resurrected and returned to power, but his stance on taking over the world seemed to have changed by then.
As it turns out, Voldemort is actually the reason that Grindelwald is dead. Once Voldemort returned to power, he made it his mission to find the Elder Wand since he believed that wielding it would make him the most powerful wizard of all time. He interrogated the wandmaker Ollivander for information about the Elder Wand, then began his search for the powerful item in Eastern Europe.
Gregorovitch had bragged about owning the most powerful wand in the world, so many suspected that he was referring to the Elder Wand. Voldemort found Gregorovitch in his store and interrogated him about the wand, only to discover that it had been stolen many years ago. Voldemort used Legilimency to read Gregorovitch's mind and saw an image of a young, blond boy escaping the wand shop with the Elder Wand in his hand. It's unclear exactly how Voldemort figured out that the blond boy was Grindelwald, but there were pictures of a young Grindelwald floating around the wizarding community after Dumbledore's family secrets were exposed in Rita Skeeter's biography about him.
Voldemort's meeting with Grindelwald in the books was slightly different in the films. In the Deathly Hallows book, Harry's mind was still connected to Voldemort, and he was able to see Grindelwald in Nurmengard prison through Voldemort's eyes. Voldemort demanded to know where the wand was, but Grindelwald taunted him by saying that the wand would never be his, welcoming Voldemort to kill him rather than giving up the location of the wand. Voldemort grew frustrated and killed Grindelwald with the Killing Curse. Harry and Dumbledore theorized that Grindelwald felt remorse about his actions of the past and wanted to make amends by keeping Voldemort from desecrating Dumbledore's tomb. Even so, Voldemort figured out that Dumbledore would have won the wand from Grindelwald in their duel and found it in the tomb anyway.
The films took a different approach that completely scrapped Grindelwald's redemption arc. Instead of protecting Dumbledore, Grindelwald said he was expecting Voldemort and gleefully told him that the Elder Wand was buried with Dumbledore. Voldemort fled the prison right away to retrieve the wand from Dumbledore's tomb, and it seemed that Grindelwald was still alive when he left. This was the only time they ever met since Voldemort died at the Battle of Hogwarts shortly afterwards.
Fantastic Beasts: The Secrets of Dumbledore is currently playing in theaters.