The X-Men #42
by John Buscema and Sam Rosen (March, 1968)
Although it appears that both Professor X and Grotesk are killed in this issue both survive. The “Charles Xavier” who died in this issue was really a disguised Changeling, who was assisting the Professor in a plot against the Z’Nox alien race which will be realized in later issues of X-Men. Grotesk would survive as well and appear in the pages of Ms. Marvel.