Nottingham Forest

One of those things that I just can't imagine happening ever again. Not just to Forest, just the notion of any newly promoted team challenging for the title is unlikely enough , but to go on to win and retain the Championship of Europe as well. Unthinkable.  
The 1976–77 season saw  Nottingham Forest finish 3rd in Division 2, claiming promotion to the top flight after a 5 season absence.
1977–78- Forest won the League Championship.
1978-79- Forest won the European Cup.
1979-80-retained the European Cup.

The European Cup was then a straight knockout tournament- home and away legs until the final- meaning Forest played 9 games to become Champions of Europe. (In 1978-79 6 wins 3 draws. In 1979-80 6 wins 1 draw  2 defeats).