Minecraft wiki entry for Creating_a_challenge_map, associated text: Make a path that goes slightly underground, then fill it to the ceiling with water. Add some air pockets so your players can breathe, using signs or open trapdoors. Spawn some squid to annoy players even more. In third party mods, you could make the squids hostile and make them shoot ink to damage and blind the players. Or you could go for a similar effect in vanilla Minecraft and have some command blocks that cause Blindness every time the player touches a squid. In addition, you could use pufferfish guardians, to deal damage, or maybe, even, a room with an elder guardian.