It’s an intraosseous ganglion.

An intraosseous ganglion (IOG) is a rare, benign, mucin-filled cystic lesion occurring within the subchondral medullary bone, commonly near joints such as the tibia, femur, or carpal bones. They often cause localized pain, though many are found incidentally. Treatment typically involves surgical curettage and bone grafting.

Sounds so good, everyone should have one. :)