documentclass{article}
usepackage{makecell}
begin{document}
begin{table}
%makegapedcells
renewcommandtheadset{defarraystretch{0.8}}%
begin{tabular}{|l|c|c|}hline
  diaghead{theadfont Diag ColumnmnHead II}{Diag Column \Head I}{Diag Column\ Head II} &
  thead{Second\column} & thead{Third\column}\
end{tabular}
end{table}
end{document}