To obtain this weapon, you have to earn enough evil points through sacrificing followers. Recruit the Henchmen that occupy Oakvale/Bowerstone South/Temple of Avo/Twinblade's Camp and sacrifice them at the Chapel of Skorm. Save the game before doing this. A threshold of 1000 evil will get you the bow.

Why can't I get Skorm's bow?

If you didn't get the bow, it's likely because your sacrifice was worth less than 600 evil points. You'll need to repeat the process again (or ideally just load the save you made just before the sacrifice). If you get at least 150, you can just re-load your save and do it again until you get the bow.

How do you get Skorm's bow before wasp queen?

Get Skorm's Bow Before Wasp Queen

You can get it using this method before you even kill the Wasp Queen. In order to do this, you must do everything you can to be good. When you have 4 bars of goodness, go to the Chapel of Skorm with a mercenary. Wait until it is a quarter or so into night, and save.