Darth Plagueis's knowledge of the dark side allowed his conciousness to exist through the force, much like yoda and obi wan exist and can manifest themselves as force ghosts. Plagueis conceived Anakin Skywalker in the hopes that he would be able to possess Anakin once he mastered the force and turned to the dark side, granting Plagueis a powerful new body to inhabit and seek revenge on Sidious for betraying him. But after Anakin's fight with Obi Wan on Mustafar, his body was no longer suitable for Plageuis, who bided his time, waiting for another opportunity. Years later, after the Galactic Civil war, Plagueis reached out through the force and seduced Ben Solo to the dark side. At the end of the Force Awakens, Snoke commands that Kylo Ren be taken directly before him to "complete his training". Snoke is going to possess the body of Kylo Ren and inhabit the body of a powerful force user once more.