A Voodoo Doll is a doll you use to curse your pet or other peoples' pets to make them get a random disease (for more info click here) and have 0% happiness.

Voodoo Dolls

But unfortunately, it is not recommended to do this; only curse your pet when you can afford to buy medicine for it,

and if you curse another person's pet, they might turn against you, but if you curse the pet of an administrator (like Ian), they will punish you in ways you can't understand.

So, tell it to the judge, 'cause your account will be banned.