Have the Mines gotten too easy for you? Have no fear - there’s another challenge for you to tackle. The Skull Cavern is a second, harder dungeon in Stardew Valley, one that will endlessly test your mettle against a slew of powerful enemies. You’ll want to be fully-outfitted before you dare to enter this place.

Entering the Skull Cavern

You need to complete two tasks before you can access the Skull Cavern:
  • First, you need to find it. The Skull Cavern is located in the northwest corner of Calico Desert, an area you can’t reach until you’ve repaired the bus from Pelican Town. This article discusses getting to Calico Desert in more detail, but the long and short is that you need either the Junimos or Joja Mart to fix the bus. Either way it’s going to cost you over 40,000 gold to get to Calico Desert.
  • Second, you need to unlock it. The Skull Cavern is barred when you first arrive, and you need the Skull Key to get in. The Skull Key is on level 120 of the Mines - in other words, the last level of the Mines. Sounds annoying, but you should be at least strong enough to get to the end of the Mines before you try the Skull Cavern. It’s much harder.
The Basics

The Skull Cavern is fairly similar to the Mines, though there are several key differences between the two:
  • There are different monsters. They are universally stronger than the monsters in the Mines, and typically faster to boot.
  • There are no elevators. Every time you enter the Skull Cavern you tart from the first level, and then go as deep as you can until you’re forced out.
  • In addition to stairs you will occasionally find pits. Hopping into a pit will drop you a random number of floors down, usually maxing out around seven or eight. Sounds good - except you take damage upon landing if you fall too far. There’s no telling how far you’ll drop, as well, and when you land you will not be beside the entrance ladder.
  • The map layouts are randomized, unlike the Mines. You won’t run through the same areas on the same levels each time you enter the Skull Cavern. (Though there are, as far as I can tell, a limited number of possible areas.)
  • The Skull Cavern is endless. I think. It doesn’t appear to have a bottom. You go until you can’t go no more.

I’m still mucking about in the Skull Cavern, so I can’t give a full rundown just yet. Nevertheless, here are some survival tips you might find handy:
  • Bring restorative items. Lots of them. The Skull Cavern is teeming with beasties, and they will whittle your health down real quick. 
  • Bring a few Bombs. Occasionally you'll run across areas populated by Mummies, and though you can slash them down they will get back up again after about twenty seconds of idling. Only by Bombing them once they're down can you destroy a Mummy for good.
  • Get the Lava Katana from the Adventurer’s Guild before making any dedicated excursions into the Skull Cavern, assuming you haven’t found a better weapon somehow. You’ll want as much firepower as you can get.
  • Speaking of the Adventurer’s Guild, several of the items you get from Gil for killing monsters are really useful down here. The Slime Charmer Ring and the Vampire Ring are equally fantastic, as one allows you to avoid Slime damage entirely (and man are there a lot of Slimes) and the other allows you to restore health by killing enemies. Very handy.
  • Max out your Combat Skill. Seriously. You want both Professions and all the bonuses you can get.
  • Be wary of pits. They’re great in theory, but a pit can really screw you up if you’re already hurting for health. Don’t drop into one unless your HP is high or you have lots of healing items.
  • Don’t be afraid to turn around and walk out. The consequences of getting beaten up in the Skull Cavern are, so far as I’ve seen, harsher than those of the Mines - mainly that you’ll lose a lot more items.