Viel zu viel `\multicolumn`
\documentclass[a4paper,11pt,oneside,numbers=noendperiod,setspace,parskip=half]{scrbook}
\usepackage[ngerman, english]{babel}
\usepackage{blindtext}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{wrapfig}
\usepackage[table,xcdraw]{xcolor}
\usepackage{array}
\usepackage{array}[2017/11/04]
\usepackage{tabularx}
\begin{document}
\blindtext
\begin{wraptable}{l}{8cm}
\bfseries
\renewcommand\tabularxcolumn[1]{m{#1}}
\newcommand\italic{\normalfont\itshape}
\begin{tabularx}{\linewidth}{|X | *{4}{Wc{.6cm}|}}
\hline
\rowcolor[HTML]{656565}
\textcolor[HTML]{FFFFFF}{Surface geometry}
& \multicolumn{4}{c|}{\textcolor[HTML]{FFFFFF}{Terrain Classes}}
\\
\hline
\italic fine texture,\par high convexity
& \cellcolor[HTML]{643403}1
& \cellcolor[HTML]{F8A102}5
& \cellcolor[HTML]{009901}9
& \cellcolor[HTML]{34FF34}13
\\
\hline
\italic fine texture,\par low convexity
& \cellcolor[HTML]{CE6301}3
& \cellcolor[HTML]{FFCC67}7
& \cellcolor[HTML]{34CDF9}11
& \cellcolor[HTML]{3531FF}15
\\
\hline
\italic coarse texture,\par low convexity
& \cellcolor[HTML]{F007F4}2
& \cellcolor[HTML]{A34894}6
& \cellcolor[HTML]{C5C783}10
& \cellcolor[HTML]{FCFF2F}14
\\
\hline
\italic coarse texture,\par low convexity
& \cellcolor[HTML]{F690E5}4
& \cellcolor[HTML]{F6C8EC}8
& \cellcolor[HTML]{DFE360}12
& \cellcolor[HTML]{FFFC9E}16
\\
\hline
\multicolumn{1}{c}{}
& \multicolumn{1}{c}{\makebox[0pt][c]{\italic steep}}
& \multicolumn{2}{c}{}
& \multicolumn{1}{c}{\makebox[0pt][c]{\italic gentle}}
\end{tabularx}
\end{wraptable}
\blindtext
\end{document}
[![alt text][1]][1]
[1]: https://texwelt.de/wissen/upfiles/test_431.png
