Submerges
From Ashenzari's Archive
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Non-adjacent monsters with the Submerges flag are able to hide or remain hidden in their current tile, even if in clear LOS of the player. A monster so hiding is indistinguishable from the terrain he occupies. While submerged, the monster may move and remain completely hidden. However, any submerged monster adjacent to the player becomes immediately revealed. Typically, this is seen when certain water-based creatures hide in water or lava-based creatures hide in lava. However, the trapdoor spider can hide on an ordinary dungeon tile (think of it as burrowing underneath the floor).