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The Nightmare in the Pathway is the first Nightmare you meet at the beginning of the game. Unlike the previous Nightmares, you'll only have 1 main encounter with this one. Make your way through the Pathway and when you'll find the Nightmare in The Summit. 

The final fight area will be made up of a blue hexagon tiled floor. To start, random hexagons will disappear and the Nightmare will toss orbs into the air. When they land, they'll fire 6 shots out from the center. He'll throw 4 orbs before the next phase.

Once he's thrown all 4, the missing hexagons will reappear and the Nightmare himself will jump down. When he lands, he will also fire 6 shots from his center. He'll jump 3 times and then jump to the center. The spots that he landed on (save for the center) will turn into spirals that will pull you in if you get too close to them. When he lands in the center, he will spin in a whirlwind attack that you'll need to dodge. After a few seconds (and if you don't get hit), he'll stop and be dizzy. You can attack him here. 

This whole process will repeat until he has but a sliver of health left. A short interactive scene will play out and the trophy will unlock.