To obtain Marshadow you must first get a code to use via Mystery Gift from a participating video game store. In the UK you'll need to head on down to your nearest GAME store. However it is worth noting that not all stores seem to have the codes yet, so you might have to wait a couple of days.
The event is running until the 16th of November, so make sure you get a hold of yours as soon as possible! The information of the Marshadow is as follows:
Fighting / Ghost
Held Item: Marshadium Z
Moves: Spectral Thief, Close Combat, Force Palm, Shadow Ball