Captain Marvel #26 begins the Thanos Saga.  This is the first time Thanos appears on a cover, and it is his 2nd full appearance.  It also contains the 1st appearance of Death, the cosmic being from the Infinity Gauntlet limited series.  Jim Starlin really outdid himself on this book (cover, interior art, and story).  This book is graded 9.6 NM+ with white pages by CGC.