The tilberi witchcraft can be only performed by women. Its goal is to steal milk from neighbors. In order to make a tilberi, a woman should go to a graveyard early on Whitsunday and dig a human rib there. Then this rib needs to be wrapped in grey wool and be preserved between breasts for three weeks. Each Sunday during this period, when the woman goes to church, she must not swallow the holy wine at the communion, but has to spit it on the bobbin she has between her breasts. Finally the tilberi (also called snakkur, ‘suckler’) gets alive and can speak: it calls its mistress ‘mamma’. Then the woman is to carve a nipple on her thigh. The tilberi hangs on it and nourishes itself. When it is fully grown, it goes to steal milk from others’ cows and sheep. It can lie across a sheep’s back and suck two tits at the same time. When it rolls back, it spurns milk into a churn. If one draws the magical sign called smjörhnútur (butterknot) on the churn, the butter made from such milk falls into small pieces.
When a woman grows old and wants to get rid from her tilberi, she has to order it to gather all the sheep-dropping in three high-land pastures. Eager to accomplish the task and come back to her mistress, the overexerted tilberi explodes. All that is left is the human rib and dropping. According to a Danish source, a woman was burnt at the stake in 1580 in Iceland for practicing tilberi magic. In 1635 another woman was acquitted after someone had accused her of creating a tilbery.