\usepackage{multirow}    % preamble
\usepackage{slashbox}    % preamble

\begin{tabular}{|l|l|l|l|}\hline
 \multirow{2}{1.3cm}{\backslashbox{a}{a}} & b & c & d\\\cline{2-4}
 & e & f & g \\\hline
 h & i & j & k\\\hline
 l & m & n & o\\\hline
\end{tabular}