A "diabolical plan" refers to an evil or wicked scheme or strategy that is cleverly designed to cause harm, chaos, or achieve selfish objectives. Full definition
Rasputin has diabolical plans for Hellboy, which includes seeing him in his rightful place as a lord of demons in an Earth taken over by the dark powers that be. (qwipster.net)
After that brilliant, diabolical plan pays off, MBC's highest honchos orders more episodes of «Sittin» Pretty» to fill the air for the next twenty - two weeks and also greenlights other programs from Vic's mind, including an animated series called «The Goombas.» (dvdizzy.com)
Is that part of the diabolical plan of the sheepherders to take over the world and control the masses? (religion.blogs.cnn.com)