.overlay {
    position: relative;
}

.overlay::before {
    opacity: 0;
    visibility: hidden;
    z-index: 1;
    -webkit-transition: all 0.3s ease;
    -moz-transition: all 0.3s ease;
    transition: 0.5s;
}

.overlay::before {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    opacity: 0.5;
    background: #000;
    content: "";
    -webkit-transform: scale(1);
    -moz-transform: scale(1);
    transform: scale(1);
    opacity: 0;
    visibility: hidden;
    -webkit-transition: all 0.5s ease;
    -moz-transition: all 0.5s ease;
    transition: all 0.5s ease;
    z-index: 35;
}

(what class do you have):hover .overlay::before {
    opacity: 0.5;
    visibility: visible;
}