CSS gradient from corner?

Question

Any ideas on how to make a corner gradient?

Using an offset conic gradient seems to give me a decent effect, but It's not quite what I'm going for.

background-image: conic-gradient(#fff 45deg, #000 215deg, #fff 270deg);

Here is a picture of the effect I am trying to get:

Answer 1

Maybe a combination of two gradients:

.box {
  width: 250px;
  aspect-ratio: 1;
  background: linear-gradient(red 20%, blue);
  position: relative;
  z-index: 0;
}

.box:before {
  content: "";
  position: absolute;
  z-index: -1;
  inset: 0;
  background: inherit; /* use the same gradient */
  transform: rotate(-90deg); /* rotate it */
  clip-path: polygon(100% 0, 100% 100%, 0 100%); /* show only half of the element */
}

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