You can complete this trophy by performing the rituals throughout the map.
If you intend to get
The Beginning of the End trophy, then this one will unlock along the way.
I have made a guide under the other trophy detailing the steps to completing the rituals. The Shadowman will appear during the rituals and will count towards the trophy.
I'm pretty sure I ended up unlocking this one during the second ritual but I can't be certain. It just randomly popped for me after the second ritual finished and I wasn't specifically going for this trophy at the time.