The Kapalikas’ kind of Tantra was considered a “left-hand” practice, meaning that it embraced impurity and occult (magic) methods of spiritual advancement.
This was in contrast to “right-hand” Tantra, which followed a more conventional form of deity yoga and meditation (using normal mantras, mandalas, and mudras).
According to legend, the Kapalikas had killed a Brahmin, and so in penance (punishment) they were sentenced to wander around as sadhus, living in cremation grounds or in forests. They had to carry the skull on their staffs to indicate their previous crime.