Marshadow is a Fighting and Ghost with the ability Technician. In Japan the event will give out Marshadow at level 50 and it will know the moves Spectral Thief, Close Combat, Force Palm and Shadow Ball. Spectral Thief is a brand new move that only Marshadow can learn and it's a move that does damage but also steals the opponents stats before the damage is dealt.
When holding the Marshadium Z Z-Crystal, Marshadow will have access to new Z-Move: Soul Stealing 7-Star Strike. There is no news on the event happening outside of Japan yet, but presumably when the move makes it overseas so will the event. You can check out a video of the new Z-Move in action above.