JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box"></div>

CSS

.box {
  width:20vw;
  height:20vw;
  background-color:#2D3142;
  background:linear-gradient(-45deg, transparent 15px, #2D3142 0) bottom right;
  filter:drop-shadow(2px 2px 10px rgba(0,0,0, 1));
}