Are you mixing up the skeleton bone with the boney monster?
In the starting cell block there is a ghost who doesn't know he is a ghost. In order to convince him, you need to go to a certain cell and retrieve a bone. This can be done before jumping down the arena pit into the haystack.
On the next level, you get a quest to find the boney monster from an imp in the kitchen. This monster can be found on this level.
You can eventually get back to the starting level, but you have to keep going forward for awhile first.
In the battlefields, the quest items and monsters are placed randomly on the third levels of the dungeons. In order to open up the second and third dungeons, you will need to find additional BF keys later in the main game. There is a second level 1 BF key in act 1; the level 2 key can be found shortly after you get to the citadel, and the level 3 key is available very near the end of the act.
A BF key will open all dungeons up to its level. If you miss the level 2 key, for example, then the level 3 key will open the second and third dungeons.
