This is a flask of ground bone meal made from human bones.
Bone Meal is created by grinding up some human bones in the mortar and pestle outside Baba Yaga's Hut in the Land of Mordavia. The Hero needs an Empty Flask in order to carry the bone meal after the bones are ground.
Bone Meal is one of the four ingredients needed to create an Elderbury Pie. A flask of Bone Meal can also be given to Baba Yaga as a bribe to gain either the Hide spell as a Wizard, the Breath Ritual, or the Good Humor Bar if the player gained a different boon when delivering the pie.