Short answers, pulled from the story.
Griffith is the main antagonist of the manga series Berserk, created by Kentaro Miura. He is the founding leader of the mercenary unit the Band of the Hawk, described as extraordinarily handsome, charismatic, and a swordsman of near-unparalleled skill. At the Eclipse, he sacrifices his mercenary comrades to become a member of a supernatural council called the God Hand, taking the name the wings of darkness.
The Eclipse is a ceremony triggered when Griffith activates a crimson behelit during a solar eclipse. During it, he sacrifices the members of the Band of the Hawk to be reborn as the final member of the God Hand. He ascends into a form called Femto, powered by the souls of those he sacrificed.
Miura stated in an interview that he did not plan Griffith's antagonism from the start and began developing it during the third volume. He chose to make the target of Guts's anger a friend, or at least a man of the same general age, because friendship mattered deeply to him personally.
Kentaro Miura stated that his friendship with manga artist Koji Mori partially inspired the relationship between Guts and Griffith. The dynamic and turbulent bond between the two characters forms the primary focus of the manga.
Falconia is a kingdom Griffith establishes as the last bastion of humanity, governed as a dictatorship. He builds it after inflicting a fatal injury on the Kushan leader Ganishka, an act that creates a global interstice, a mixing of the physical and supernatural worlds. He leads Midland's armies with the backing of the Holy See Church to reach that point.
Critics have widely praised Griffith as a complex villain. Anne Lauenroth of Anime News Network called him the perfect villain, citing his human qualities and fatal flaw. Paste magazine ranked him second on their list of the twenty greatest villains in anime. Scholars including Natalia Kucma have examined his transformation as a case study in transhumanism and the ethical costs of sacrificing subjectivity and physicality for the realization of a dream.