\documentclass{article}

\usepackage{slashbox}

\begin{document}

\begin{tabular}{|c|c|c|} \hline 
\backslashbox{unten}{oben} & b & c \\ \hline
d & 1 & 2 \\ \hline
e & 3 & 4 \\ \hline
\end{tabular}


\end{document}