You have to sacrifice 5 people to the Temple of Skorm.
The number of people sacrificed doesn't matter. The time does. The tricky thing is that the correct time changes based on your alignment. Neutral sacrifices at midnight, good and evil sacrifice towards 6 AM and PM, but I'm not sure which is which. The easiest way to get the bow is, immediately after you leave the heroes guild (or whenever your alignment is neutral), go to Bowerstone, to unlock it as a teleport option. Don't do anything in Bowerstone, just enter and leave. Then go to the Temple of Skorm in Darkwood. Use your guild seal to teleport from the Temple of Skorm to Bowerstone and hire the mercenary in the bar. After you've hired him, use the recall option to teleport directly back to the Temple of Skorm. Mercenaries can teleport with you. Then, world save and wait until midnight.