It's also known as kapala (Sanskrit for "skull") or skullcup, it is made from a human skull used as a ritual implement (bowl) in both Hindu Tantra and Buddhist Tantra (Vajrayana). Especially in Tibet, these skulls often carved or elaborately mounted with precious metals and jewels. The heavily embossed cup is usually made of silver-gilt bronze with lid shaped like a skull and with a handle made in the form of a thunderbolt.
|a Tibetan skull for rituals|
And the design development from the idea of Tibetan skull is vary, including on Dimas Praja's tattoo work:
|a Tibetan skull tattoo by Dimas Praja, done in 2009|