Add div content to my already existing menu

So your links are all set static? If I don’t miss something badly then why do you need the wp_nav_menu-function then at all. You code could look like so:

<div class="menujeu">
<div class="dropdown">
<button class="dropbtn">HACK & SLASH
<i class="fa fa-caret-down"></i>
</button>
<div class="dropdown-content">

<a href="https://www.dm-gaming.eu/chronicon/">Chronicon</a>
<a href="https://www.dm-gaming.eu/diablo-iii/">Diablo 3</a>
<a href="https://www.dm-gaming.eu/diablo-4/">Diablo 4</a>
<a href="https://www.dm-gaming.eu/dreamscaper/">DreamScaper</a>
<a href="https://www.dm-gaming.eu/grim-dawn/">Grim Dawn</a>
<a href="https://www.dm-gaming.eu/Hammerwatch/">HammerWatch</a>
<a href="https://www.dm-gaming.eu/hero-siege/">Hero Siege</a>
<a href="https://www.dm-gaming.eu/Last-Epoch/">Last Epoch</a>
<a href="https://www.dm-gaming.eu/Lost-Ark/">Lost Ark</a>
<a href="https://www.dm-gaming.eu/path-of-exile/">Path of Exile</a>
<a href="https://www.dm-gaming.eu/path-of-exile-2/">Path of Exile 2</a>
<a href="https://www.dm-gaming.eu/warhammer-chaosbane/">Warhammer Chaosbane</a>
<a href="https://www.dm-gaming.eu/Wolcen/">Wolcen</a>

<!-- Here your 3rd closing div was missing-->
</div>
</div>
</div>

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