Gellert Grindewald was a wizard who was tried of the mistreatment that the wizarding community suffered at the hands of the humans, wanted to kill all humans so the wizarding community could rule the Earth.
To that end, he located the Maia known as Credence Barebones and egged him on, feeding Credence's desire for revenge. However, when Credence unleashed his powers, he didn't care about Gellert's agenda and just wanted to cause mass destruction and carnage.
Gellert was eventually arrested and sent to Azkaban, swearing revenge all the way.
Later, he escaped from Azakaban, reunited with Credence, and resumed his schemes for world domination.Years later, he finally got his chance for revenge when Lord Voldemort and the Death Eaters freed him from Azkaban, knowing that Gellert had useful information about the Deathly Hallows. However, once Gellert told Voldemort everything he knew about the Deathly Hallows, Voldemort, having no further use for Gellert, killed Gellert.