Vertical line in a table with merged cells

Here is a solution based on the multirowpackage:

\documentclass{report}
\usepackage{tabularx}
\usepackage{multirow}
\usepackage{xcolor}

\newcommand{\customrulesep}{\vrule width 2pt}
\newcommand{\rulesepOverOneRow}{\multirow{1}{*}{\customrulesep}}
\newcommand{\coloredRulesepOverOneRow}{\multirow{1}{*}{\color{blue}\customrulesep}}

\begin{document}
    \begin{tabularx}{\textwidth}{l c X}
        Description & \coloredRulesepOverOneRow & Lorem ipsum dolor sit amet, consectetur adipiscing elit. Morbi at mi non nunc interdum molestie. \\
        & & multilignes\\
        Validation & \rulesepOverOneRow & Ceci est un texte \\
        & & multilignes\\
    \end{tabularx}
\end{document}

You can adapt the thickness by changing width 2pt to any dimension you like.

Result

Enjoy!

I propose this variant, with a slightly simpler code, and a completely different solution, vased on a customised description environment:

\documentclass{report}
\usepackage{tabularx}
\usepackage[svgnames, x11names]{xcolor}

\usepackage{eqparbox}
\usepackage{enumitem}
\newenvironment{mydescription}[1][descr]{%
\renewcommand*{\descriptionlabel}[1]{\eqmakebox[{#1}][l]{##1}\hskip1.25em\textcolor{NavyBlue!60}{\rule[-0.8ex]{1pt}{2.5ex}}}%
\description[labelsep=1.25em, leftmargin= \dimexpr\eqboxwidth{#1}+1em+\labelsep + \fontdimen 2\font]}%
{\enddescription}

\begin{document}

 \noindent \begin{tabularx}{\textwidth}{@{}l@{\hskip1.25em\textcolor{NavyBlue!60}{\rule[-0.8ex]{1pt}{2.5ex}}\hskip1.25em}X}
        Description & Lorem ipsum dolor sit amet, consectetur adipiscing elit. Morbi at mi non nunc interdum molestie. \newline multilignes\\
        Validation & Ceci est un texte \newline multilignes\\
    \end{tabularx}
\bigskip

    \begin{mydescription}
        \item[Description] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Morbi at mi non nunc interdum molestie. \newline multilignes\\
       \item[Validation] Ceci est un texte \newline multilignes\\
    \end{mydescription}

    \begin{mydescription}[descra]
        \item[Long description] Lorem ipsum dolor sit amet, consectetur adipiscing elit. Morbi at mi non nunc interdum molestie. \newline multilignes\\
       \item[Validation] Ceci est un texte \newline multilignes\\
    \end{mydescription}

\end{document}