Normally box-shadow is static. By throwing the card into 3D CSS Space using perspective: 1000px, we can tilt the card toward the mouse, exposing a massive array of layered inner shadows imitating an endless deep tunnel.

Stacking Inset Shadows

.portal-card {
  transition: transform 0.3s, box-shadow 0.3s;
  box-shadow: inset 0 0 0 #000; /* Flat */
}

.portal-wrapper:hover .portal-card {
  /* Creating the glowing abyss tunnel */
  box-shadow: 
    inset 0 0 10px #7e22ce,
    inset 0 0 30px #4c1d95,
    inset 0 0 60px #312e81,
    inset 0 0 100px #000;
  
  /* Tilt into the screen */
  transform: perspective(600px) rotateX(10deg) rotateY(-10deg) translateZ(-50px);
}

Check the Live Demo. The box physically opens a glowing purple dimension directly into your monitor!