documentclass[11pt]{article}
usepackage{multirow}
usepackage[table]{xcolor}

begin{document}
begin{center}
begin{table}[!htbp]
noindent
begin{tabular}{|c|p{3cm}|p{3cm}|p{3cm}|}
hline
multirow{2}{3cm}{centering cellcolor{gray!50} Effectiveness of Controls} &    multicolumn{3}{p{9cm}|}{centering cellcolor{gray!50} Probability of Threat Occurrence (Natural or Environmental Threats) or Threat Motivation and Capability (Human Threats)} \
cline{2-4}
rowcolor{gray!50}
& Low & Moderate & High \
hline
rowcolor{white!50}
cellcolor{gray!50} Low & Moderate & High & High \
hline
rowcolor{white!50}
cellcolor{gray!50} Moderate & Low & Moderate & High \
hline
rowcolor{white!50}
cellcolor{gray!50} High & Low & Low & Moderate \
hline
end{tabular}
caption{Risk Classifications}
label{tab:xyz}

end{table}
end{center}
end{document}

multirow{2}{dimexpr0.25textwidth-2arrayrulewidth-2tabcolseprelax}[-0.5cm]{...

documentclass[11pt,draft]{article}
usepackage{multirow}
usepackage[table]{xcolor}
usepackage{array}
newcolumntype{C}[1]{>{centeringarraybackslash}p{#1}}

begin{document}
begin{center}
begin{table}[!htbp]
noindent
begin{tabular}{|c|C{dimexpr0.25textwidth-arrayrulewidth-2tabcolseprelax}
           |C{dimexpr0.25textwidth-arrayrulewidth-2tabcolseprelax}
           |C{dimexpr0.25textwidth-arrayrulewidth-2tabcolseprelax}|}
hline
multirow{2}{dimexpr0.25textwidth-2arrayrulewidth-2tabcolseprelax}[-0.5cm]{centering cellcolor{gray!50} Effectiveness of Controls} &    multicolumn{3}{p{dimexpr 0.75textwidth-arrayrulewidth-2tabcolseprelax}|}{cellcolor{gray!50}centering Probability of Threat Occurrence (Natural or Environmental Threats) or Threat Motivation and Capability (Human Threats)} \
cline{2-4}
cellcolor{gray!50}
& cellcolor{gray!50} Low & cellcolor{gray!50} Moderate & cellcolor{gray!50} High \
hline
%rowcolor{white!50}
cellcolor{gray!50} Low & Moderate & High & High \
hline
rowcolor{white!50}
cellcolor{gray!50} Moderate & Low & Moderate & High \
hline
rowcolor{white!50}
cellcolor{gray!50} High & Low & Low & Moderate \
hline
end{tabular}
caption{Risk Classifications}
label{tab:xyz}

end{table}
end{center}
end{document}

documentclass{article}
usepackage{cals, caption, xcolor}

letnc=nullcell           % CALS Shortcuts for spanning cells
letsc=spancontent


begin{document}

begin{table}[!htbp]
begin{calstable}[c]  % Centred CALS tabular

% Calculate the column width
colwidths{{dimexpr(columnwidth)/4relax}
            {dimexpr(columnwidth)/4relax}
            {dimexpr(columnwidth)/4relax}
            {dimexpr(columnwidth)/4relax}
    }

% The tabular fills the text area if sum of all columns is 4

% Set up the tabular
makeatletter
defcals@framers@width{0.4pt}   % Outside horizontal frame rules
defcals@framecs@width{0,4pt}   % Outside vertical frame rules
defcals@bodyrs@width{0.4pt}    % Rule between header and tabular body
defcals@cs@width{0.4pt}        % Inside vertical rules
defcals@rs@width{0.4pt}        % Inside horizontal rules
defcals@bgcolor{}              % To avoid undefined

defgray{ifxcals@bgcolorempty     % "Switch" to turn on and off colour
    defcals@bgcolor{gray!30}
else defcals@bgcolor{} fi}

% R1H1     % Just a comment: Row one, header row one
thead{
brow
    grayalignCnc{lrt}    % Switch to gray background and centred text, works until switched off
    nc{ltb}
    nc{tb}
    nc{rtb}sc{vfil Probability of Threat Occurrence (Natural or Environmental Threats)}
erow
%R2H2       % Row two, header row two
brow
    nc{lrb}sc{vfil Effectiveness of Controls}  % "vfil" centres the cell content vertically
    cell{vfil Low}
    cell{vfil Moderate}
    cell{vfil High}gray % Always reset colour at end of header
erow
%R3B1     % Row three, body row one
brow
    graycell{Low}gray   % Switch to gray background for first cell (column 1)
    cell{vfil Moderate}
    cell{vfil High}
    cell{vfil High}
erow
%R4B2
brow
    graycell{Moderate}gray
    cell{vfil Low}
    cell{vfil Moderate}
    cell{vfil High}
erow
%R5B3
brow
    graycell{High}gray
    cell{vfil Low}
    cell{vfil Low}
    cell{vfil Moderate}
erow
makeatletter
end{calstable}par
caption{Risk Classifications}
label{tab:xyz}
end{table}
end{document}