* { margin: 0; padding: 0; box-sizing: border-box; }
.nav-bar {
width: 100%; background: #444; height: 100px; position: relative;
}
.nav-bar .nav-bar-menu-btn {
width: 100px; height: 100px; background: blue;
float: right; text-align: center;
color: #fff;
cursor: pointer; transition: all 0.4s ease;
}
.nav-bar .nav-bar-menu-btn:hover { color: red; }
.nav-bar .nav-bar-menu-btn:hover .mega-menu { color: #fff; }
.nav-bar .nav-bar-menu-btn .mega-menu li:hover { color: red; }
.header-banner { background: #18bc9c; width: 100%; height: 2350px; }
.mega-menu {
position: absolute; top: 0; right: 0;
width: 100%; height: auto; background: #2c3e50;
transition: all 0.4s ease;
padding: 40px;
visibility: hidden; opacity: 0;
}
.inner-mega-menu { width: 24%; float: left; list-style: none; }
.inner-mega-menu li { padding: 10px 0; text-align: left; transition: all 0.4s ease; }
.nav-bar .nav-bar-menu-btn:hover .mega-menu {
visibility: visible; opacity: 1;
}
<div class="container" style="position: fixed; width: 100%;">
<nav class="nav-bar">
<div class="nav-bar-menu-btn">Menu
<div class="mega-menu">
<ul class="inner-mega-menu">
<li>Bibendum</li>
<li>Mattis Ridiculus</li>
<li>Ligula</li>
<li>Purus Sem</li>
</ul>
<ul class="inner-mega-menu">
<li>Bibendum</li>
<li>Mattis Ridiculus</li>
<li>Ligula</li>
<li>Purus Sem</li>
</ul>
<ul class="inner-mega-menu">
<li>Bibendum</li>
<li>Mattis Ridiculus</li>
<li>Ligula</li>
<li>Purus Sem</li>
</ul>
<ul class="inner-mega-menu">
<li>Bibendum</li>
<li>Mattis Ridiculus</li>
<li>Ligula</li>
<li>Purus Sem</li>
</ul>
</div>
</div>
</nav>
</div>