Letter from Delilah is a written note found in Dishonored 2.
You ask what the Void looks and feels like, if it can be measured like a real place. Here's my answer: Don't concern yourself with such matters. It is as real as anything I've ever experienced, but if you understood it, you'd know that such a statement makes as little sense as saying that I have been dead.
The Void is unspeakable. It is infinite and it is nowhere, ever-changing and perpetual. There are more things in the endless black Void, Kirin Jindosh, than are dreamt of in your natural philosophy.
Leave aside things beyond your reach, and be content that you are gifted with more insight than the common man.