Uncanny X-Men, Vol. 1 #72 (1971)

From Whence Comes... Dominus?
The X-Men attack Lucifer in his home base, where they are faced with his Dominus machine and the robots who maintain it. His plan is to use the machine to dominate the Earth, but the X-Men can't defeat him until they manage to agree on whether or not the Professor's warnings came of his own free will, or at Lucifer's behest.
Release Date: 1971