Demonic Spiders/Sandbox Games

Even in wide open worlds, there's nearly Everything Trying to Kill You and no one else.

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• Grand Theft Auto III has every gang pack a lot of heat, but the worst of them come from the mafia. After betraying Salvatore, wandering into mafia's territory will cause them to shoot you on sight with shotguns. Shotguns in the game are capable of killing anyone in one or two shots and when you got several people with shotguns trying to kill you, it will take a miracle for you to get out unscathed. Driving in a car? Shotguns are so powerful that the mafia can almost make your car instantly explode in a few shots with their shotguns. Hope you weren't planning to do side missions after turning against the mafia!
• Minecraft has several mobs that make your life a living hell:
  • Creepers. Walking plants that don't make any noises until they get close enough to you to hiss and then explode. They can cause massive damage if you are not wearing armor, wreck any structures you built, and destroy items. Creepers are also the only way you can obtain the rest of the music discs by having a Skeleton kill it. Creepers will also stay away if you have a cat nearby, but ocelots are incredibly rare to find in the first place, which you tame to get a cat.
  • Ghasts. They only appear in the Nether but they will make your time in the Nether constantly dangerous. They can float and will usually float out of range of your swords, making them being vulnerable to arrows. Ghasts also constantly shoot fireballs at you that explode on contact, causing major damage unless you are wearing armor. There's also an achievement for sending a Ghast's fireball attack back at it, but good luck trying to get the Ghast to sit still for it to be hit.
  • Cave Spiders, being a literal form of this trope. They only appear in abandoned mineshafts and attack like a regular spider, but if you are playing on Normal or higher, their attacks will poison you, causing your health to drop at a constant rate. They are very similar to the Poison Headcrabs where the poison and the Cave Spider cannot kill you directly, but the poison itself can leave your health at just half a heart, making you a One-Hit-Point Wonder to other enemies or pitfalls. A bucket of Milk can cure the poison, but you have to hope there aren't more Cave Spiders nearby to poison you again. To make matters worse, the spiders are not slowed down by cobwebs.
• For most of Terraria The Corruption torments you with erratically-moving monsters that hit like a truck and swarm in numbers; worse, the Corruption area itself grows constantly and displaces other environments. By the time you're able to reach the Underworld, though, the Corruption is a forgotten memory and Corrupted regions have probably been healed, sealed, or blown to atoms with tons of dynamite. Then you enter the endgame ... and the Corruption gets souped up and starts spreading all over again. Fortunately, its opposite number, the Hallow, appears to battle it. Unfortunately the Hallow is even worse.