You do not have to visit the other cave on the Plateau by force, but it will
give you your first powerful weapon. The cave is far to the west, by a
dilapidated building with a chest inside. The cave will present many
traps, so keep on the run.Going inside the cave from the Herdsman's Valley, you are traversing your first challenge of fighting and solving puzzles in order to get onto the Plateau and meet Mythran.
Following the path in the cave, you come to water and have to jump over stepping stones. Without the patch, it is very easy to fall and die, so you might have to do it about 20 times. To the west is a trapped chest with some scrolls. Following the stepping stones, go a little further than the chest and jump onto the west bank, then follow it until you have to climb onto higher ground. Do so, and you will come to an electric gateway, which you can evade by climbing the even higher ground. If you try taking the passage behind the elevated ground, you will be killed by fire shrooms. This elevated ground has some items you might find useful - there is a death disk under a skull, a dagger on the floor and the skeletons contain potions and a gem - the only catch is that a ghoul guards this high ground so you'll have to kill it to take the stuff.
I discovered a bug that works if you can manage to jump to the south bank thanks to the extra range permissible by the patch; if you do so and go by the wall west, you will end up underground. You can skip the entire cave if you can find the right way to the exit. You will have to go beyond the exit into some void and go back, otherwise you will not be able to get out of the ground. It is very wierd, but cool enough. If you remain in the void and run away from the area, you can make your way to other areas. I managed to get to the Plateau, Mythran's house and the Bonecrusher cave with this - you can reach any of the areas which are part of the Plateau (see the "Going Beyond Area Boundaries" section of the Hidden Things page for full details).
Assuming you did not make this illegal entry to the Plateau, turn west and climb up. The bodies on the floor sometimes have potions or gems. Skip the lever and continue west across the bridge. A skeleton here has arm guards and a backpack with 2 flasks of oil. Pull every lever that does not have any bones in front of it (the screen will shake if you did it right), then run back across the bridge and pull the lever you skipped earlier. This will open the gate, and you can get through the cave by going south.
By following the path south, you will reach Mythran's house. If you go far to the west, you will come to an abandoned building, near which is the cave that contains the Bonecrusher. The abandoned building is guarded by changelings, and the chest inside has a helmet and a flask of oil. Jump to the chest, however, if you intend to open it, as the ground in front of it will collapse under your weight and drop you on red mushrooms. There is also a patch of adventurer's mushrooms by the water just east of the ruined building if you want to take some healing with you - they will be most useful if you venture into the Bonecrusher Cave where you will undoubtedly get hurt a lot. The mushrooms in and around the ruined building, on the other hand, are poisonous so don't eat them. There are a couple of other chests in the area, guarded by seekers and ghosts, but they do not contain anything important. When you feel like, drop by Mythran's house.
Just run through the traps and you will not be hit. Find Mythran and speak to him, telling him you seriously need to leave Pagan. Mythran is an important character, so talk to him and learn all you can. When you leave him, he will give you a potion and the Recall Item. Mythran should send you to the Necromancers. Make sure you visit his Recall Pad on the second floor of his house, and, when he tells you that you can take his stuff, go on the ground floor and help yourself to Korghin's Fang, which is in the backpack in the room with 2 kegs. The books on the second floor contain much useful history about Pagan.
You do not have to visit the other cave on the Plateau by force, but it will
give you your first powerful weapon. The cave is far to the west, by a
dilapidated building with a chest inside. The cave will present many
traps, so keep on the run.
Your first obstacles will be fire shooters, and after a troll, you will have to pass more fire shooters and a spitter.
You will eventually come to a hall filled with dead bodies, and here you will be assailed by some invisible beings, so watch out for their red eyes. At the end of this hall is a pair of locked double-doors, thought by many to be the entrance to the Lost Vale. There is no way to open them; I have tried Mythran's secret door scroll (which you encounter later on in the game), I have searched every skeleton's body and I've tried everything; there is no way to open them.
If you go to the end of the cave, you will find adventurer's mushrooms (which you'll probably need badly once you get the Bonecrusher), after which there will be a chest which contains the Bonecrusher, which will be the Hammer of Strength if you don't have the patch. It is a very good weapon, so take it with you. There is more good stuff in the corridor where the Bonecrusher is found, including:
The corridor is guarded by a skeleton, however, so it is difficult to take everything as the skeleton will keep coming to life even after you kill it (this is a major source of injuries if you intend to take all the stuff).
Mythran, and probably Bentic, directed you to the Necromancer. With your new Recall Item, you can beam back to Central Tenebrae in an instant if you bothered to activate the Recall Pad as I warned you. If not, you will have to go all the way back through the cave again.
Either way, go to east Tenebrae and continue east until you come to the East Road. Here, you can embark upon an optional subquest or else continue north to the Cemetery. The choice is yours, as long as you get to the Cemetery at the end of the subquest.