Monsters with the Sumberges flag are hidden from the player, until they become adjacent to you for the first time. While hidden, submerged monsters are undetectable, even if in clear LOS of the player.
This is usually seen when monsters dip below water or lava, where they can only remain submerged while they are on a water/lava tile (respectively). Trapdoor spiders can hide inside ordinary dungeon tiles, however (think of it as burrowing underneath the floor).
- The submerging mechanic was removed in 0.22.
- Prior to 0.18, monsters with this flag could resubmerge if the player managed to escape their notice.