Spoiler: DC 15 Perception
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The swamp seems darker than before, the nearby trees have become dark twisting shadows with clawlike branches reaching out towards you. A foul odor rises up from the murky water. Overhead through the twisted branches even the sky looks strange. The stars look out of position and the moon is much too close than it should be, nearly full. Menacing clouds are rolling in behind you. Like a curtain of black smoke. It takes no time for the clouds to blot out the stars, casting you into further darkness.
Spoiler: Perception DC 20
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The swamp is watching you. You know it, shadows slithering out of sight at the edge of your peripheral vision. The sound of frogs is omnipresent, but off... that loud song of frogs isn't a chorus of a multitude of frogs.... its a few.... loud frogs, louder than you've ever heard.
Spoiler: Perception DC 25
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You can see bats moving in the shadows, rats jumping from cypress to cypress, snakes and crocodiles in the water. Most of these are normal swamp creatures, but there is something menacing, something vile in their eyes.
Spoiler: knowledge planes
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You are definitely not on the same plane anymore