The Yekubians are a fictional race created in 1994 by Chaosium for its Call of Cthulhu RPG, based on H.P. Lovecraft's The Challenge from Beyond.
The Yekubians are a race of worm-creatures whom invade worlds and exchange their minds with those of the natives in a manner similar to the Great Race of Yith. They employ 4 inch square crystalline cubes and any intelligent creature who looks into the cube, has its mind drawn into a Yekubian machine. The vacated body is taken over by a Yekubian mind which uses it to explore. Once it's done, it uses the cube to send its own mind back.