Monster Stone

Monster Stones can be looted from random drops (very rare) or are also able to be collected through completing Achievements. They are items that can be assigned to your hotbar and used. When you use it, you are given 2 options:
 * Enter
 * Receive EXP

Enter
If you choose to enter the monster's nest, you will be given 30 minutes to get to the end and defeat the final boss which will then automatically teleport you back out upon being beaten.

When inside, you will see many monsters that have a high amount of health but a fairly average attack damage, for the effort of killing them you can collect materials for Combining Old Accessories and a significant amount of EXP per kill. You can either choose to kill them all or simply bypass them to get to the final boss quickly (a method commonly used by INT Priests who aren't equipped to deal damage).

Receive EXP
This option is available if you do not wish to spend time in the monster's nest, it will allow you to immediately claim a chunk of EXP although it is only a fraction of what you can gain from killing the monsters so it's rarely a good choice to make.