<div></div>

div {
  background-color: blue;
  width: 100px;
  height: 100px;
  clip-path: polygon(
    0 0, /* top-left */
    100% 0, /* top-right */
    100% 100%, /* bottom-right */
    0 100%, /* bottom-left */
    25% 50% /* 25% in left, 50% down */
  );
}