In "Terraria," The Guide Voodoo Doll is dropped only by the Voodoo Demons in the Underworld. The Guide Voodoo Doll is an item used to kill the Guide NPC and summon the Wall of Flesh. Equipping the Guide Voodoo Doll accessory allows the player to attack and kill the Guide NPC in "Terraria."
Voodoo demons carry the Guide Voodoo dolls and drop them when killed. If the doll falls into lava in the Underworld, the final pre-Hardmode boss, the Wall of Flesh, is instantly summoned and the Guide NPC is killed. The Guide NPC must be alive when the doll is destroyed or the Wall of Flesh is not summoned. The doll must be dropped in the Underworld. Dropping the Voodoo Doll in lava in the other world layers does not summon the Wall of Flesh.
Only one Wall of Flesh can be summoned at a time. Throwing multiple dolls into the lava at once wastes the extra Voodoo Doll items.
Various other NPCs, as well as the Guide NPC himself, give clues on how to find the Guide Voodoo Doll. The Guide NPC says that you have to make a sacrifice to find the Guide Voodoo Doll, suggesting the Guide NPC must be killed.