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Thank you!
Add linear-gradient after the parent container using Pseudo-Element :after starting form the bottom with 100% and fade top 0%.
Check the code below:
body {
  margin: 0;
  padding: 0;
}
.main {
  background-image: url(https://picsum.photos/1280/1000?image=1080);
  background-size: cover;
  background-position: center center;
  height: 100vh;
  position: relative;
  height: 100vh;
  width: 100wh;
  overflow: hidden;
}
.effect {
  width: 100wh;
  height: 100vh;
}
.effect:after {
  display: block;
  position: relative;
  background-image: linear-gradient(to bottom, rgba(0, 0, 0, 0) 0, #000 100%);
  margin-top: -100vh;
  height: 100vh;
  width: 100wh;
  content: "";
}<div class="effect">
  <div class="main">
  </div>
</div>