You can create an internal shadow in CSS by using the box-shadow property with the “inset” keyword. Here’s an example:
```
.box-with-internal-shadow {
box-shadow: inset 0 0 10px #000000;
}
```
In this example, the box-shadow property is applied to an element with the class `box-with-internal-shadow`. The `inset` keyword makes the shadow appear inside the element instead of outside. The three values after `inset` define the X offset, Y offset and the blur radius of the shadow, respectively. The hash value `#000000` is the color of the shadow, in this case, black.
To make the shadow spread larger and softer, adjust the third value for the blur radius and the fourth value for the spread radius such as:
```
.box-with-internal-shadow {
box-shadow: inset 0 0 20px 10px #000000;
}
```
