It's called a Keystone. Thousands of years ago, my people sealed dangerous Pokemon into them, to prevent them from causing trouble. I learned my blood magic by unsealing this one.

[Merrill taps on the stone, releasing the Spiritomb inside. Its single eye swirls ominously inside the vortex of light and glowing orbs that comprise its body. The same swirling pattern can be seen in one of Merrill's own eyes.]