Grendel Prime breaks into the complex where Jupiter Niklos Assante resides. His knows of the structures security measures as well as the codes to make his way through the building. Prime kills all of the guards protecting the children, and takes Jupiter with him. Jupiter's step-mother Laurel Kennedy arrives at the compound to find the destruction and devastation caused by Prime. She dispatches a squadron of Red Devils in pursuit of the abductor. Prime easily slays the Red Devils and moves through Chicago. Kennedy continues to track their progress as they make their way toward New York City. Outside of New York city Grendel is stopped by the Guardians of Pain, a gang who controls entrance to the city now known as "the pit". In order to appease them Grendel makes a gesture of pain by hurling himself off a bridge, bouncing off the rocky shores into the river below. The guardians allow the pair entrance into the city. Prime and Assante travel through the pit, encountering toxic mutant zombies as well as robot extermination drones. They leave the city, hoping that moving through the city has convinced Kennedy that they are dead.
Their detour through the pit proved to be in vein as Laurel Kennedy continued to track them down to the Louisiana bayous. They are chased through the swamps by a squadron of Red Devils. Assistance comes from an old swamp witch whose illusions convince the Red Devils and Laurel Kennedy that Prime and Jupiter died during an explosion in the swamps. Prime makes his way to a freighter at the docks and makes his way out to sea.
On the ocean the freighter is chased down by a pirate ship. Prime knew that the ship's crew couldn't hold their own against a pirate attack, and planned a diversion to gain the element of surprise. Prime killed four members of the ship's crew and spread their blood amongst the rest of the crew who lay on the deck pretending to be dead. The pirates boarded to find what looked like a mass suicide and Prime sprung into action, killing the pirates. Prime and Jupiter make their way through the ocean alone. He kills the rest of the crew and blows up the freighter in hopes of continuing to keep Kennedy off their tracks.
After two weeks of exposure on the open water, Grendel and Jupiter finally make it to the African coast. Grendel is physically exhausted and delirious as he makes his way to a river to provide water for the pair. Just upstream a tranquilizer is applied to the water so that Prime and Jupiter may be easily captured. Because of his current state it takes Prime much longer to recover from the tranquilizer than the child. He is greeted by the leader of a tribe that remains loyal to Grendel Kahn. After proving himself loyal, Prime is allowed to continue on his journey with Jupiter. He disguises Jupiter as a local and they make their way towards the mountains. They are chased by the red devils up to the foot of the mountain where they break off. Prime must fight off another tribe and a mountain gorilla before being able to proceed, but at this point their cover is blown. The world will know that Grendel Prime and Jupiter Assante are still alive and in the African jungle.
While traveling through the OPEC nuclear wastelands Prime and Assante stopped at a ruined stone building to retrieve fresh water from a well. They are taken captive by a group of mutants that blame technology for all of their problems. Grendel Prime, weakened from the radiation, was unable to fight all of them off and was nearly executed. He was saved by the men of Azif A Barouk, leader of a group of rebels that oppose the rule of Grendel's widow Laurel Kennedy Assante. Prime awakes underneath the ruins of the Vatican to find himself partially disassembled and paralyzed from the neck down. His captures know of Jupiter's identity and do not believe the story that he is Orion Assante's personal paladin. Prime activates a holographic projection of Orion which gives the bearer permission to do whatever he needs in the name of the Grendel Kahn. While Azif and his rebels are deciding what to do with Prime a group of vampires kills Jupiter's guards and kidnap Jupiter. When news gets to Azif, Grendel Prime is reassembled and recharges himself with their generators. The group sets out for Siberia, where the vampires came from.
After battling the royal guards to get out of the Vatican, Prime and the remaining rebels travel through the Siberian desert. There they are buried under an avalanche and must dig themselves out before continuing. By this point prime's power source is extremely depleted by exertion. They locate Jupiter's signal at an abandoned missile silo, and must battle through the vampire hoards before getting Jupiter back. Within the missile silo they locate the still-functional power generator. Grendel Prime is able to recharge himself, making him completely battle-ready again. The team kills most of the vampires during the course of the rescue including an old enemy of past Grendels, Pellon Cross. Jupiter is taken to safety. That evening they watch on the television as Laurel Kennedy's role is taken over by her first in command. The young Jupiter realizes that this is not the appropriate time to re-appear, and they remain in hiding in the wilderness.
Ten years would pass and in that time Jupiter would build the power of the rebel army. Grendel assisted him until the time when their plans would come to fruition, and Jupiter would seize the throne that was rightfully his. They make their way through the bearing straight, and through the Canadian wilderness. It was there that they discover Jupiter's step-sister, Crystal Kennedy. Crystal had escaped her mother with her bodyguard/lover Susan Veraghen around the time Jupiter was kidnapped. They traveled back to north America with Crystal and Susan. With the help of the rebels, Grendel infiltrated the sun house which contained Orion Assante's legendary weapon the Sun Disc. Within Grendel Prime's mechanical body Orion Assante had built the components required to activate the power of the sun disc. Prime used this to reactivate the sun disc and destroyed a satellite orbiting the Earth, giving Jupiter access to the worldwide communication network he would need to proclaim his status as Grendel Kahn.
After helping to take back the throne Grendel left. The duty he was charged with was to assist Jupiter in seizing control, and now his job was finished. He rode off, never to be seen by Jupiter again.
Grendel Prime is a cyborg created by Orion I. His mechanical body is solar powered in nature, but he can recharge himself using electrical power sources. His body possesses superhuman strength and durability. He is generally invincible when fully powered, however if he is unable to recharge himself he will lose power through exertion. When Prime's power is low he is more susceptible to injury and defeat by enemies.
He is an expert at unarmed combat, capable of fighting many foes simultaneously. Prime is also adept with many forms of weapons, particularly his powered sword and shield and handguns. Within Prime's mechanical body Orion also built in the necessary components to operate the Sun Disc, an extremely powerful weapon commissioned by the Khan.
Grendel Prime often uses a laser sword in combat.
In the Batman crossover, he shows numerous abilities. He can make his shoulder spikes stick out of his body like short spears. He can survive being hit by a tractor trailer and shrugs off standard bullets. He can kick through the bottom of a bridge and can easily punch through police bulletproof shields. He is resistant to electrical charges, although they can knock him out of they are powerful enough.
He can create a solar flare around his body that can melt most materials, although this drains him of much of his energy.
If he is disassembled, he can remotely control his separated body parts.
In the second Batman/Grendel crossover, Grendel Prime travels back to modern Gotham City. He appears in a museum exhibit on mass murderers which includes the skull of Hunter Rose. Commissioner Gordon and the police are there to guard the exhibition, and he attacks them, viciously killing thirteen of them easily. Batman arrives. He blasts Batman in the chest with a police shotgun, and they fight. Batman covers him with some solidifying goo, but Grendel Prime melts his way out of it. This lowers his energy supply, so he jumps out of the building.
Batman chases him. Grendel Prime is hit by a tractor trailer but isn't injured. He kicks his way through the bottom of the bridge and leaps to the river far below. Later, it procures a large stock of weapons from local gangs. Then it goes to a Wayne electronics building which Batman (realizing that he is solar-powered) has advertised to be working on advanced solar technology. Prime tries to hook himself up to the energy source but doesn't have the passwords. He finds an employee and has him do it for him.
Right after he is re-powered, Batman shows up. He catches him with an electrified net design specifically to resist Grendel Prime's powers, but he cuts his way out with his laser sword. They fight again, and he beats Batman badly, breaking his arm and knocking him out.
He forces the hacker employee to leave with him, and then to help him build a time machine so he can return to his own time. This process will also perpetuate a mysterious explosion and blood sacrifice that will help him communicate with the spirit of Hunter Rose.
Batman later builds some technology which shorts him out in their third fight. Batman takes him apart and finds his organic brain still inside. He scans his computer memory, learning that Grendel Prime lived for centuries after Jupiter's reign, continually improving his own design.
Grendel Prime rebuilds himself and escapes. He manages to get Hunter Rose's skull from the museum, and is about to complete the ritual when he is interrupted by Batman. Robin shoots the skull out of his hands with an arrow, leaving a large hole in the forehead (which is later seen in in-continuity Grendel issues). Grendel Prime manages to return to his future without completing the ritual, and Batman takes possession of Rose's skull, placing it in the Batcave.