Is Nikon D810 and Canon EF 800mm f/5,6 L IS USM compatible?

Below you will find your chosen camera and lens. Click on the product name for more information.

Buy	Nikon D810 Released: 2014 Type: Digital SLR Sensor: FX (Full frame) 36 Megapixels 7360 x 4912 pixels
Buy	Canon EF 800mm f/5,6 L IS USM Released: 2008 Telephoto Lens (prime) Focal Length: 800 mm Aperture: f/5,6

Can you combine these two?

This is the short answer:

No! This combination is not compatible

The camera and lens you have chosen cannot be combined. Here is a longer answer:

A non-Nikon lens on a Nikon camera

This is a Canon lens and unfortunately they are not constructed to work with Nikon cameras. If the lens would have been a Sigma, Tamron och Tokina it would perhaps be different since they are available with a compatible Nikon mount.