It was a Skill Rank Stone dungeon.
Skill Rank Stones and Skill Point Stones are resources that can be used to increase the ranks of the skills.
One directly increases the rank of the skill, while the other gives Skill Points, which are used to increase the rank of a skill.
The issue was that both have grades. The low-grade one is used to improve a skill up to Rank-3, and after that, only Mid-grade and High-grade ones work.
The low-grade ones may not work on higher ranks, but a higher grade skill rank stone would work on the lower ranks. So, there is always demand for the high-grade ones, especially because they are the rarest of all grades.
There are dungeons, but the problem was that only the boss from the dungeon would drop such an item, while no other items or EXP would be gained for clearing other monsters. It was a waste of time for many.
