  The bulk of X-ray spectroscopic studies of active galactic nuclei (AGN) are
focused on local ($z < 0.1$) sources with low-to-moderate ($< 0.3$) Eddington
ratio ($\lambda_\mathrm{Edd}$). It is then mandatory to overcome this
limitation and improve our understanding of highly accreting AGN. In this work
we present the preliminary results from the analysis of a sample of $\sim70$
high-$\lambda_\mathrm{Edd}$ radio-quiet AGN at $0.06 \leq z \leq 3.3$, based on
the 10th release of the XMM-Newton serendipitous source catalogue, that we
named as XMM-Newton High-Eddington Serendipitous AGN Sample (X-HESS). Almost
$\sim35\%$ of the X-HESS AGN have multi-epoch archival observations and
$\sim70\%$ of the sources can rely on simultaneous OM optical data. First
results reveal sources showing signatures of ultra-fast outflows and remarkable
long- and short-term X-ray flux variations. Indeed in J095847.88+690532.7 ($z
\sim 1.3$), one of the most densely monitored objects hosting a
$\sim$$10^9\,M_\odot$ supermassive black hole, we discovered a variation of the
soft X-ray flux by a factor of > 2 over approximately one week (rest-frame).
Large variations in the power-law continuum photon index $\Gamma$ are also
observed, questioning expectations from previously reported $\Gamma -
\lambda_\mathrm{Edd}$ relations, for which $\Gamma \geq 2$ would be a
ubiquitous hallmark of AGN with $\lambda_\mathrm{Edd} \sim 1$.
