Cant position ::before Pseudo Element behind its parent

I tried to create a div, and have its ::before appear behind it to make it seem as some sort of shadow, but for some reason I cant get the pseudoelement to appear behind the original element.

<div class="hero container split">
                <div class="hero__cont-image bg-dark"><img src="" alt="" class="hero_cont-image__image">
                </div>

.hero__cont-image {
  position: relative;
  transform: rotate(0);
  display: flex;
  align-items: center;
  justify-content: center;
  left: calc(-50vw + 50%);
  border-radius: 0px 300px 300px 0px;
}

.hero__cont-image::before {
  content: "";
  background-color: blue;
  position: absolute;
  width: 125%;
  height: 125%;
  z-index: -1;
}

I want the pseudo element to appear behind the original element

Solution

On the parent div transform will create a new stacking context making the parent div the root element of the stacking context. A stacking context’s root element will always be the lowest, even behind elements with negative z-index. The psuedo-element can't lower itself to be below the root element even with negative z-index, hence will always be above the background.

If your are not using transform remove it and the pseudo element with negative z-index will appear to be below the parent since now the parent is not the root element of their stacking context anymore.

.hero__cont-image {
  position: relative;
  display: flex;
  align-items: center;
  justify-content: center;
  left: calc(-50vw + 50%);
  border-radius: 0px 300px 300px 0px;
}

.hero__cont-image::before {
  content: "";
  background-color: blue;
  position: absolute;
  width: 125%;
  height: 125%;
  z-index: -1;
}

<head>
  <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-T3c6CoIi6uLrA9TneNEoa7RxnatzjcDSCmG1MXxSR1GAsXEV/Dwwykc2MPK8M2HN" crossorigin="anonymous">
  <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.2/dist/js/bootstrap.bundle.min.js" integrity="sha384-C6RzsynM9kWDrMNeT87bh95OGNyZPhcTNXj1NW7RuBCsyN/o0jlpcV8Qyq46cDfL" crossorigin="anonymous"></script>
</head>

<body>
  <div class="hero container split">
    <div class="hero__cont-image bg-dark"><img src="https://i.sstatic.net/memI0.png">
    </div>
  </div>
</body>

Note: These elements creates new stacking context: Opacity,transforms, filters, css-regions, paged media. If you need to rotate the parent element, create a wrapper div on the div and apply the transforms on the wrapper instead.