The God Hand all started as humans who like Apostles sacrificed their loved ones to become demons. However, the God Hand were chosen specifically chosen by the Idea of Evil to serve as its purpose for suffering. Every 216 years, a Crimson Behelit falls into the hands of a human to sacrifice a large group of their followers in order to transcend their humanity in a ceremonial event known as the "Eclipse." Their Behelit is also called the "Egg of the King". All of the God Hand are worshipped by Apostles, even the most powerful of them must submit to their authority. The God Hand are so powerful they cannot easily take form in the human world. They usually enter through short lived constructs made from anything they want.
The five current members are:
Void: The leader, and has to do with the destiny of all.
Griffith (or Femto): Can warp space.
Slan: Clearly lust and pain.
Ubik: Likely has to do with dreams and manipulation.
Conrad: He seems to represent death and disease.
Just like every other Berserk character up to chapter 364, the God Hand were created by the late Kentaro Miura.
The God Hand also seem to have been inspired by Hellraiser according to a conducted interview.
Interviewer: "Is there something you used as a reference when you created Berserk’s world with your imagination?"
Miura: "There are many things. Movies like "Hellraisers" and "The Name of the Rose". I've liked Escher for a very long time. Well, I think Berserk readers would already know this kind of thing from "behind the scene" features... It's also inspired by Grimms' Fairy Tales and so on."
In Chapter 362, a flashback is shown. The previous wearer of Guts' Berserker Armor is looking at an old variation of the God Hand, the only member who is in the new God Hand is the current archangel, Void.
The fate of the old God Hand is shrouded in mystery as of Chapter 369.
Major Story Arcs
The Golden Age
The shadows of the God Hand initially appear in the Golden Age before Griffith during his year long torture. They tell him that they await his presence.
Not too long after this scene happens, Griffith re-obtains his Crimson Behelit as the Band of the Hawk come to rescue him. Once his blood comes in contact with it, he and the Hawks are pulled to another dimension where the astral and physical worlds become temporarily connected.
Each member of the God Hand makes their appearance one-by-one, before telling the Hawks their true reason for being here: To be sacrificed. To avoid their interference, Griffith alone is raised to the sky and brought to the God Hand where they have a private talk without the Hawks interfering.
Ubik looks into Griffith's mind and convinces him that if he does not sacrifice the Band of the Hawk, all that he's worked for will forever be lost and his dream won't be accomplished, that if he repents for all the soldiers that have died under his command, he will join the corpses of the soldiers that he has built up.
Griffith is convinced and agrees to sacrifice the Hawks, being secluded to be reborn. The Hawks are marked with the Brand of Sacrifice. After the Hawks are massacred by the Apostles, Griffith is reborn as the God Hand member "Femto." Before his former comrades Guts and Casca could be finished off, the mortal enemy of the God Hand, the Skull Knight barges into the event and saves them despite Femto's attempt to kill him.
Black Swordsman Arc
During this time, Guts has gone on a vengeful killing spree against the Apostles of the God Hand. In a tough battle, Guts has defeated an Apostle named "The Count," who has called to the God Hand while on the verge of death. Guts becomes angry at the sight of Griffith and tries to attack but is telekinetically pushed away before he could attack.
The Count attempts to bargain for his life. At first, he attempts to offer Guts to the God Hand to save himself, but he is denied as not only has Guts been offered by Griffith before, but he is also not someone dear to The Count. The God Hand suggest that he sacrifice his daughter, Theresia. The Count declines. Since he was on the verge of death he is dragged to the Vortex of Souls, a domain of tortured souls where all Apostles, those who bear the Brand of Sacrifice, and those who are involved with demons in general go to.