The abyssal whip is one of the best weapons in the game because of its consistancy, it has a quick attacking rate and also can hit high.
The abyssal whip can be obtained through slayer, to do so you will need to have 85 slayer and the combat level (high combat) to kill and abyssal demon, They abyssal demons are located in the Slayer Tower on the 3rd floor. The whip is a rare drop and can take some time to obtain.
The requirements for the whip, are just simply 70 attack. nothing else is needed.