This is how it will work (although some things might change).
There will be a few new maps that are instanced, and the following conditions will apply:
1. One player can go there every so often, which is defined by each instance type. The time between going there again will be something like 100-200 hours.
2. Most, if not all of the instances will have minimum and maximum a/d requirements, so for example a pr0 player won't be able to go and spoil the fun for the newbies.
3. Each instance type will require a certain number of players to go in (a minimum and maximum number).
4. There might be a cost associated with going in an instance.
5. Most (or all) of the instance monsters will be unique monsters, with unique drops.
6. To join an instance, a group of players will have to join a secret channel that they chose. Then they will all go to an NPC which will teleport them to the desired instance. To prevent abuse, all the players from that channel must stay near the NPC.
7. If you die in an instance, and you have a rostogol stone, you will be placed at the beginning of the instance, so that you can rejoin your friends.
8. There will be 5 (or more) monster waves, that is, if you kill a wave, the next will spawn shortly after. Those waves will be somewhat random, from a predefined set of possibilities. The final wave will be a 'boss' type of monster.
9. The drops will be unique items that can not be obtained in any other way. My current plan is to make some really powerful items, but they will have a high chance to degrade, so the game won't be full of them.
That's pretty much it. I don't know exactly when they will be in the game, probably in a month, if everything goes well.