"Collect Witch Doctor Mask" is an objective in the sidequest Victims of Vault Hunters in Sir Hammerlock vs. The Son of Crawmerax.
Walkthrough[edit]
After Sparky Flynt finishes talking at you, inspect the Sacrificial Totem. Sir Hammerlock will contact you at that point and explain what you need to get: a Witch Doctor Mask. Head back across the bridge to the North and into the Sanctum of the Sacred Insect.
Once in that area, you should have little trouble finding a Witch
Dialogue[edit]
Sir Hammerlock: Ah, yes -- I've seen one of these totems before. You'll need to offer a sacrifice to get past it. I recommend a mask from one of their witch doctors -- I'm sure those chaps wouldn't mind loaning you one.