Grendel's forked staff is his primary weapon.
Hunter Rose originally chose the design in order to further the demonic imagery associated with his alias. In his usual style, Rose added enhancements to the staff that would make it even more deadly. This staff would get passed down to Christine Spar years after Rose's death. I was broken in her final fight with Argent.
The fork is perhaps three or four feet long, thin, and light. The double-forked blade is undetectable by metal detectors or any other security measures, yet its steel blade is razor sharp. The blade itself is retractable, and can be completely hidden in the hilt. In this way he can keep the staff with him at all times as a walking stick, and can still use it as a weapon whenever he chooses.
It can also fold up into three equally-sized sticks, which he can carry around in a briefcase.
Circuitry within the staff allows the blade to be electrified with the touch of a button, but the charge doesn't last very long: perhaps a few minutes. The charge is enough to kill someone, or light flammable materials on fire.
Its blunt end is hard and can be used as a weapon as well; Christine Spar often rammed people in their face or mouth with it, breaking their teeth.
There is also a radio transmitter located in the staff so he can use it for communication.
The staff has been shown to have at least two options for firing a retractable cable off the end. Christine Spar used it to fire off just the blades, which she then used as a climbing too. Hunter Rose used fired the cable from the blunt end, which seemingly has a short sharp tip that can stick into target surfaces. He used this cable to set up tripwires. Also, he could throw the entire fork at someone, with cable in tow, and then retract the fork back up to him.
Eppy Thatcher's staff
A similar staff was crafted hundreds of years in the future by crazy genius Eppy Thatcher to be used as a part of his own Grendel persona. Unlike Rose's staff, Thatcher's blade was not retractable (as Thatcher did not use the staff when he was not Grendel). Thatcher's staff used an extremely hard blade, and had technology incorporated into it that allowed him to override other technology such as the church's automated collection plates.
He could still electrify the blade, and also built a laser into the other end, which he used to cut holes through tough materials.