You can spot the Shadowman six times during a single game of zombies. The first opportunity to spot him is when you spawn at the start of the game, he will be above you and you can spot him clearly, the other five times is during the rituals.

If you are going to do all five rituals then you will unlock this trophy. If you are going to do only the first four rituals then don't forget to spot the Shadowman at the start of the game so you can earn this trophy after the 4th ritual end. Take a look at  The Beginning of the End for more details.