What object is used to summon the title being in 'The Haunter of the Dark'?

In “The Haunter of the Dark,” the Shining Trapezohedron is used to summon the title being.

Robert Blake discovers the artifact in a disused church on Federal Hill in Providence. It is a strange stone set inside a metal box, whose alien markings and hinged lid conceal an object with occult powers. Looking into it allows Blake to perceive distant places and beings, but his investigation also releases the Haunter.

The entity can move freely only in darkness, making the church’s lighting a temporary barrier. During a thunderstorm, the city’s electrical power weakens and eventually fails, allowing the creature to approach Blake’s rooms. Blake dies while staring toward the church, and a doctor later throws the trapezohedron into Narragansett Bay.

The object is sometimes confused with other Lovecraftian artifacts, especially the Silver Key, which belongs to Randolph Carter’s Dream Cycle stories. The Shining Trapezohedron is distinctive to this tale and is linked to Nyarlathotep in later interpretation. Lovecraft wrote the story in November 1935; Weird Tales published it in December 1936.