After unlocking:
Head right through the gate you opened, and enter the door in the background (that is shadowy black). Walk left and climb down 2 ladders. Head left and climb up the next ladder. Walk into the next room to your left and you'll see a glowing blue hole in the ceiling. Press
to turn into a ghost and fly through it. Float left, then down through another hole. Possess the glowing lever and flip it to open the door.
Press
and walk left to grab (
) the key in the newly opened room. Go back down the ladder you climbed to get up here and enter the background-shadowy door on your left. Head down the ladder on your left, then keep going left and open the door using your key. Walk left again until you see a glowing hole in the ceiling, then turn into a ghost and fly through it. Possess and flip the lever. Then, reenter your body.
Backtrack all the way through the locked door you opened, climb back up the ladder, and enter the shadowy door. Walk all the way right until you can climb down a ladder. Do so. Head all the way left until you see a key.
Be ready to move kinda quick. Grab the key, and run all the way right. Turn into a ghost and squeeze through the glowing hole. Possess and flip the switch, then turn back into a human and escape up the ladder.
Walk a little left and Flying Hawk will talk to you for a sec. Achievement unlocked!