Context : The XXL Survey is the largest homogeneous and contiguous survey carried out with XMM-Newton .
Covering an area of 50 deg ^ { 2 } distributed over two fields , it primarily investigates the large-scale structures of the Universe using the distribution of galaxy clusters and active galactic nuclei as tracers of the matter distribution .

Aims : Given its depth and sky coverage , XXL is particularly suited to systematically unveiling the clustering of X-ray clusters and to identifying superstructures in a homogeneous X-ray sample down to the typical mass scale of a local massive cluster .

Methods : A friends-of-friends algorithm in three-dimensional physical space was run to identify large-scale structures .
In this paper we report the discovery of the highest redshift supercluster of galaxies found in the XXL Survey .
We describe the X-ray properties of the clusters members of the structure and the optical follow-up .

Results : The newly discovered supercluster is composed of six clusters of galaxies at a median redshift z \sim 0.43 and distributed across \sim 30 \arcmin \times 15 \arcmin ( 10 \times 5 Mpc ) on the sky .
This structure is very compact with all the clusters residing in one XMM pointing ; for this reason this is the first supercluster discovered with the XXL Survey .
Photometric redshifts from the CFHTLS ( Canada-France-Hawaii Telescope Legacy Survey ) data release T0007 placed the supercluster at an approximate redshift of z _ { phot } \sim 0.45 ; subsequent spectroscopic follow-up with WHT ( William Herschel Telescope ) and NTT ( New Technology Telescope ) confirmed a median redshift of z \sim 0.43 .
An estimate of the X-ray mass and luminosity of this supercluster returns values of 1.7 \times 10 ^ { 15 } M _ { \odot } and of 1.68 \times 10 ^ { 44 } erg s ^ { -1 } , respectively , and a total gas mass of M _ { gas } =9.3 \times 10 ^ { 13 } M _ { \odot } .
These values put XLSSC-e at the average mass range of superclusters ; its appearance , with two members of equal size , is quite unusual with respect to other superclusters and provides a unique view of the formation process of a massive structure .

Conclusions :