That is natural veining in Luna Pearl granite. While LP is a fairly homogenous tightly speckled stone, it can have light or dark veining running through some slabs. And that is what that veining looks like. You also have a darker shadow vein to the right. You didn't see this when you went to select the slabs? Or discuss it during the placement of the template onto the stone?

Agree it's natural. But also unfortunate because it LOOKS like a repaired crack.

And @Minardi if the realtor is explaining what the line is, it could easily be that the OP was there for stone selection and templating.

It sounds like the OP is considering buying this house, so not involved in the slab selection.

It's very possible that the person who made decisions for this kitchen liked the vein (though I might have tried to put it somewhere other than the corner).

100% natural but you don’t like it. Does it kill the house for you?

If that's a repair, that person is an artist. Can you feel it? I doubt you can. If not that's even more evidence that it's natural.