You can download the demo via the Xbox Store right now:
Here's a bit more information on what you can expect from the demo, along with screenshots:
"The exciting new demo gives players early access to the first two chapters of Lies of P, including challenging boss encounters, introductions to key characters, and unique areas to explore.
The demo features hours of gameplay, and allows players to freely explore the game’s primary hub area – the Hotel Krat. The demo offers an introductory look at Lies of P, but only scratches the surface of the game’s full content fans can look forward to at launch."