Monster stones are special instances where you can get very nice amount of experience. You have also chance to drop old accessory from final boss.
Monster stones are dropped randomly from all monsters (adequate to your level - for example level 83 will not drop it from worm) and you can get some as achievement reward!
To see, what achievements give monster stone as reward, visit KO Bugda's achievement list
There are few types of Monster Stone instances (based on your level) and each one has different amount of mobs inside. Here, you can see how much exp you will get on each level of each instances and can better decide on what level you will try to complete them.