The Voodoo Demon is a monster that spawns in The Underworld. It either attacks by melee, or by ranged, casting 4 Demon Scythe projectiles. The Voodoo Demon's sprite has a Guide Voodoo Doll attached to it, which drops when the Voodoo Demon is killed 100% of the time. Caution is advised when fighting the Voodoo Demon, as the boss Wall of Flesh spawns when the Guide Voodoo Doll is dropped in lava in the underworld, thus the player must be aware of it's location before killing it.
Terraria - How to KILL THE GUIDE
Notes
If a Voodoo Demon were to be killed from this position, then the Wall of Flesh would spawn
Due to the risk of summoning the strongest pre-hardmode boss (Wall of Flesh) if it is killed over lava, the Voodoo Demon could be considered the most dangerous pre-hardmode monster. But direct-threat wise, it won't be so.
The Voodoo Demon's hitbox, oddly, includes the doll below, which is not very realistic since it's not a part of the enemy itself. This might be due to programming limitations.